Thiol-selective water-soluble polymer derivatives

ABSTRACT

The present invention provides water-soluble, polymer derivatives having a thiol-selective terminus suitable for selective coupling to thiol groups, such as those contained in the cysteine residues of proteins, as well as methods for preparing the water-soluble, polymer derivatives having a thiol-selective terminus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/681,357, filed Nov. 19, 2012, now U.S. Pat. No. 8,722,032, which is acontinuation of U.S. patent application Ser. No. 12/909,755, filed Oct.21, 2010, now abandoned, which is a continuation of U.S. patentapplication Ser. No. 10/753,047, filed Jan. 6, 2004, now U.S. Pat. No.7,910,661, which claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 60/438,555 filed Jan. 6, 2003 and to U.S.Provisional Patent Application Ser. No. 60/455,084, filed Mar. 14, 2003,the disclosures of each of the foregoing are incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a method for preparing thiol-selectivederivatives of a water-soluble polymer such as polyethylene glycol. Inparticular, the invention relates to: (i) a method for preparingpolymers having a thiol, protected thiol, or other group suitable forcoupling to the thiol group of a protein or other active agent at atleast one terminus, (ii) the thiol-selective polymers themselves, (iii)conjugates thereof, and (iv) methods for utilizing such polymers.

BACKGROUND OF THE INVENTION

Due to recent advances in biotechnology, therapeutic proteins and otherbiomolecules, e.g. antibodies and antibody fragments, can now beprepared on a large scale, making such biomolecules more widelyavailable. Unfortunately, the clinical usefulness of potentialtherapeutic biomolecules is often hampered by their rapid proteolyticdegradation, instability upon manufacture, storage or administration, orby their immunogenicity. Due to the continued interest in administeringproteins and other biomolecules for therapeutic use, various approachesto overcoming these deficiencies have been explored.

One such approach which has been widely explored is the modification ofproteins and other potentially therapeutic biomolecules by covalentattachment of a water-soluble polymer such as polyethylene glycol or“PEG” (Abuchowski, A., et al, J. Biol. Chem. 252 (11), 3579 (1977);Davis, S., et al., Clin. Exp Immunol., 46, 649-652 (1981). Thebiological properties of PEG-modified proteins, also referred to asPEG-conjugates or pegylated proteins, have been shown, in many cases, tobe considerably improved over those of their non-pegylated counterparts(Herman, et al., Macromol. Chem. Phys., 195, 203-209 (1994).Polyethylene glycol-modified proteins have been shown to possess longercirculatory times in the body due to increased resistance to proteolyticdegradation, and also to possess increased thermostability (Abuchowski,A., et al., J. Biol. Chem., 252, 3582-3586 (1977). A similar increase inbioefficacy is observed with other biomolecules, e.g. antibodies andantibody fragments (Chapman, A., Adv. Drug Del. Rev. 54, 531-545(2002)).

Typically, attachment of polyethylene glycol to a drug or other surfaceis accomplished using an activated PEG derivative, that is to say, a PEGhaving at least one activated terminus suitable for reaction with anucleophilic center of a biomolecule (e.g., lysine, cysteine and similarresidues of proteins). Most commonly employed are methods based upon thereaction of an activated PEG with protein amino groups, such as thosepresent in the lysine side chains of proteins. Polyethylene glycolhaving activated end groups suitable for reaction with the amino groupsof proteins include PEG-aldehydes (Harris, J. M., Herati, R. S., PolymPrepr. (Am. Chem. Soc., Div. Polym. Chem), 32(1), 154-155 (1991), mixedanhydrides, N-hydroxysuccinimide esters, carbonylimadazolides, andchlorocyanurates (Herman, S., et al., Macromol. Chem. Phys. 195, 203-209(1994)). Although many proteins have been shown to retain activityduring PEG modification, in some instances, polymer attachment throughprotein amino groups can be undesirable, such as when derivatization ofspecific lysine residues inactivates the protein (Suzuki, T., et al.,Biochimica et Biophysica Acta 788, 248-255 (1984)). Therefore, it wouldbe advantageous to have additional methods for the modification of aprotein by PEG using another target amino acid for attachment, such ascysteine. Attachment to protein thiol groups on cysteine may offer anadvantage in that cysteines are typically less abundant in proteins thanlysines, thus reducing the likelihood of protein deactivation uponconjugation to these thiol-containing amino acids.

Polyethylene glycol derivatives having a thiol-selective reactive endgroup include maleimides, vinyl sulfones, iodoacetamides, thiols, anddisulfides. These derivatives have all been used for coupling to thecysteine side chains of proteins (Zalipsky, S. Bioconjug. Chem. 6,150-165 (1995); Greenwald, R. B. et al. Crit. Rev. Ther. Drug CarrierSyst. 17, 101-161 (2000); Herman, S., et al., Macromol. Chem. Phys. 195,203-209 (1994)). However, many of these reagents have not been widelyexploited due to the difficulty in their synthesis and purification. Forinstance, the method of Woghiren, et al. (Woghiren, C., et al.,Bioconjugate Chem., 4, 314-318 (1993)) requires a series of synthetictransformation and purification steps to form a particularthiol-protected PEG reagent. First, methoxy-PEG is reacted with tosylchloride followed by a purification of the reaction product to recoverthe corresponding tosyl-PEG. Tosyl-PEG is then converted to thecorresponding PEG-thioacetate by reaction with a thioacetate salt,followed by another purification step. Alcoholysis with methanol is thencarried out on the PEG-thioacetate, followed by column chromatography toyield the purified thiolate salt, which is then reduced withdithiothreitol to form the corresponding PEG-thiol. The resulting PEGthiol is then purified by column chromatography. A protected form of thethiol is then prepared by reaction of the PEG-thiol with 4,4′-dipyridyldisulfide, followed by purification by column chromatography. In sum,Woghiren's methodology for transforming PEG to its thiol-protected formrequires five different reaction steps and an additional five separatepurification steps, making this and other similar synthetic approachesundesirable and extremely time-consuming.

Another significant deficiency in many of the existing routes tomonofunctional thiol specific PEG derivatives is the inability, despitemultiple purification steps, to remove difunctionalized PEG which arisesfrom the diol that is present in the monofunctional PEG raw material.

Thus, there exists a need for a method for preparing high purity,activated PEG-thiols and other thiol-selective PEG derivatives that isboth straightforward and simple, i.e., requiring a minimum number ofreaction and purification steps, whilst maintaining the integrity of thePEG segment (i.e., is carried out under mild reaction conditions), andwhich can further provide high purity thiol-selective PEG derivatives inhigh yields. Such a method has been developed by the Applicants, to bedescribed in greater detail below.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method for preparing athiol-selective derivative of a water-soluble polymer (i.e., a polymerhaving at least one terminus that is a thiol-selective group, that is tosay, a group that reacts preferentially with thiols such as a thiol, athiolate, or a protected thiol). More particularly, the method includesthe steps of (i) providing a water-soluble polymer, comprising a watersoluble polymer segment designated herein as “POLY”, having a terminusactivated with an electrophile (-E), designated generally herein as“POLY-E”, followed by (ii) reacting such polymer with a reactantmolecule comprising both a nucleophile (-NU) and a thiol-selectivemoiety under conditions effective to promote reaction between theelectrophile and the nucleophile to form a water-soluble polymer havinga thiol-selective terminus, designated herein as “POLY-S”, where “S” inthis usage indicates a thiol-selective moiety. Thiol-selective moietiesthat may be contained within the reactant molecule include thiol,protected thiol, disulfide, maleimide, vinyl sulfone, iodoacetamide, andorthopyridyl disulfide.

In one particular embodiment of the invention, the thiol-selectivemoiety is selected from the group consisting of thiol, protected thiol,and disulfide.

In instances in which the thiol-selective moiety contained in thereactant molecule is a disulfide bond, the method may further comprisethe step of reducing the disulfide bond in POLY-S to form awater-soluble polymer having a terminal thiol group (designated hereinas “POLY-SH”).

In one embodiment of the invention, e.g., when the polymer provided instep (i) is an end-capped linear polymer or amono-functionally-activated polymer having only one reactiveelectrophilic terminus, the POLY-S product composition from step (ii)comprises greater than about 95% mole percent of the desiredmono-functionally substituted POLY-S. Preferably, in this embodiment,the polymer provided in step (i) is a polyalkylene oxide containing lessthan about 5% of a combination of polyalkylene oxide diol and/or abifunctional electrophilically-activated derivative of a polyalkyleneoxide diol, based upon overall polymer components.

In yet another related embodiment, e.g., when the starting polymer is anend-capped or monofunctionally-activated polymer having only onereactive electrophilic terminus, the POLY-S product from step (ii)comprises less than about 5% di-functionally-substituted POLY-S.

In a further embodiment, the polymer in step (i) comprises anelectrophile (-E) that is a carboxylic acid or an activated carboxylicacid derivative. Such electrophiles include carboxylic acid, amide,carboxylic acid ester, carbonate ester, carbonic acid, acid halide, andanhydride. In one specific embodiment of the invention, the polymer instep (i) is an N-hydroxysuccinimidyl propionate or anN-hydroxysuccinimidyl butanoate derivative of polyethylene glycol.

In yet another embodiment, the electrophile, E, is a carboxylic acid oran activated carboxylic acid derivative, and POLY-E, or a precursorthereof, is purified prior to the reacting step. Preferred methods ofpurification include chemical and chromatographic methods. In apreferred embodiment of the method, POLY-E is purified by columnchromatography prior to the reacting step. In yet a more specificembodiment, POLY-E is purified by ion exchange chromatography or IEC.

The reactant molecule includes a nucleophile (-NU). Suitablenucleophiles include primary amino, secondary amino, hydroxy, imino,thiol, thioester, and their anionic counterparts where applicable. In aparticular embodiment, the reactant molecule comprises a nucleophilethat is a primary or secondary amino group.

In yet another embodiment, the reactant molecule is a symmetricaldisulfide reagent comprising identical nucleophiles (-NU), generally asend groups, wherein the reacting step results in the formation of asymmetrical polymer having a central disulfide bond. In a specificembodiment, the molecule for reaction with the electrophilicallyactivated polymer is cystamine or cysteamine. Alternatively, thereactant molecule is N-(2-amino-ethyl)-3-maleimido-propionamide,optionally protected as an amine salt.

In a further embodiment of the method wherein the thiol-selective moietyis a thiol, the method may further comprise the step of reacting thePOLY-S thiol with a thiol or protected thiol group of a protein to forma disulfide-linked polymer-protein conjugate, designated generallyherein as POLY-S—S-protein.

The invention further provides a disulfide-linked polymer proteinconjugate produced by such method.

In another aspect, the invention encompasses a water-soluble polymerhaving a thiol-selective terminus produced by the above method,designated generally herein as POLY-S. Illustrative thiol-selectivegroups include thiol, protected thiol, disulfide, maleimide, vinylsulfone, α-haloacetyl compounds such as iodoacetamide and iodoacetate,mercurials, aryl halides, diazoacetates, and orthopyridyl disulfide.

Water-soluble polymer segments suitable for use in the invention includepolyvinylpyrrolidone, polyvinylalcohol, polyacryloylmorpholine,polyoxazoline, and polyoxyethylated polyols.

In a preferred embodiment of the invention, the polymer is apolyalkylene oxide such as polyethylene glycol (PEG).

A polymer of the invention may further comprise an end-capping groupsuch as C₁-C₂₀ alkoxy, preferably methoxy, ethoxy or benzyloxy.

In yet another embodiment of the invention, the polymer, e.g., apolyethylene glycol polymer, has a nominal average molecular massselected from the group consisting of from about 200 to about 100,000daltons, or from about 200 to about 60,000 daltons, or from about 500 toabout 40,000 daltons. In a preferred embodiment, the polymer has amolecular weight ranging from about 20,000 to about 40,000 daltons. Onepreferred polyethylene glycol polymer has a molecular weight of about20,000 daltons.

Polymers suitable for use in methods and compositions of the inventionmay possess a number of different geometries including linear, branched,forked and multi-armed. Polymers having a linear structure includemono-functional, homodifunctional and heterobifunctional polymers.

In yet another embodiment, a polymer segment for use in the inventionmay comprise a hydrolyzable linkage.

In another aspect, the invention provides a method for preparing a thiolderivative of a water-soluble polymer that includes the following steps.Step (i) comprises providing an electrophilically-activated polymer,designated herein specifically asPOLY-L_(0,1)-E   (I).In the preceding structure, POLY is a water-soluble polymer segment, Lis an optional linker, where L₀ indicates the absence of a linker and L₁indicates that such a linker is present, and E is an electrophile. Step(ii) comprises reacting POLY-L_(0,1)-E with a symmetrical disulfidereagent, designated more specifically herein as (NU-Y—S—)₂, wherein NUis a nucleophile, Y is a group interposed between “NU” and thethiol-selective group, in this case a disulfide, and S is a sulfur atom,under conditions effective to promote reaction between E and NU tothereby formPOLY-L_(0,1)-X—Y—S—S—Y—X-L_(0,1)-POLY ((POLY-L_(0,1)-X—Y—S—)₂),   (II),wherein X is a group resulting from the reaction between E and NU.Preferred Y groups are selected from the group consisting of alkylene,substituted alkylene, cycloalkylene, substituted cycloalkylene, aryl,and substituted aryl, comprising from about 2 to about 10 carbon atoms.

In another embodiment, the method may further include the step of (iii)reducing the disulfide bond in (POLY-L_(0,1)-X—Y—S—)₂ to formPOLY-L_(0,1)-X—Y—SH,   (III),where “—SH” is a thiol.

Particular L's contained in the polymer include C₁-C₁₀alkyl and C₁-C₁₀substituted alkyl. In one embodiment, the linker is selected from thegroup consisting of (CH₂)_(1, 2, 3, 4 and 5).

Representative E's include carboxylic acids or activated carboxylic acidderivatives such as carboxylic acid, carboxylic acid ester, amide,carbonate ester, carbonic acid, acid halide, and anhydride. In oneparticular embodiment, E is a succinimidyl ester.

NU's in the symmetrical disulfide reagent include amino, hydroxy, imino,and thiol. In one specific embodiment, NU is —NH₂.

Resulting X groups contained in POLY-L_(0,1)-X—Y—S—S—Y—X-L_(0,1)-POLY((POLY-L_(0,1)-X—Y—S—)₂) include amide, carbamate, carbonate ester,ether, and thioester.

In one embodiment, POLY comprises the structure —(CH₂CH₂O)_(n)CH₂CH₂—wherein n ranges from 10 to about 4,000, preferably from about 20 toabout 1,000.

In yet another embodiment, POLY is an end-capped polyalkylene oxide suchas polyethylene glycol, L is L₀ or —CH₂—, and E is N-hydroxysuccinimidylester.

In yet another embodiment, the symmetrical disulfide reagent iscystamine, where NU is primary amino and Y is —(CH₂)₂—.

In yet another aspect, the invention provides a method for preparing apolymer-protein conjugate, said method comprising the steps of: (i)providing an electrophilically-activated polymer, POLY-L_(0,1)-E,wherein POLY, L, and E are as previously defined, (ii) reactingPOLY-L_(0,1)-E with a symmetrical disulfide reagent, (NU-Y—S—)₂, whereinNU, Y, and S are as previously defined, under conditions effective topromote reaction between E and NU to formPOLY-L_(0,1)-X—Y—S—S—Y—X-L_(0,1)-POLY ((POLY-L_(0,1)-X—Y—S—)₂), whereinX is a group resulting from the reaction between E and NU, (iii)reducing the disulfide bond in (POLY-L_(0,1)-X—Y—S—)₂ to formPOLY-L_(0,1)-X—Y—SH, and (iv) reacting POLY-L_(0,1)-X—Y—SH with a thiolor protected thiol group of a protein to form a protein conjugate,POLY-L_(0,1)-X—Y—S—S-protein,   (V).

In one embodiment of the above method, the protein is a therapeuticprotein.

In yet another aspect, the invention provides an activated polymercomprising the structure:POLY-L_(0,1)-C(O)G-Y—S—W,   (VI)

In structure VI, G is a heteroatom selected from the group consisting ofO, —NH, —NR² where R² is lower alkyl, and S, and W is H or a protectinggroup. The remaining variables are as previously defined. In structureVI, —C(O)G- is a particular embodiment of “X”.

Linkers, L₁, for use in the activated polymer include aliphatic linkersof from one to ten carbon atoms. Particular linkers include(CH₂)_(1, 2, 3, 4 and 5).

In one particular embodiment of this aspect of the invention, POLY is anend-capped polyethylene glycol, L is absent or is —CH₂—, G is —NH, and Yis (CH₂)₂.

Also provided herein are compositions comprising the above describedpolymers and their conjugates.

In yet another aspect, the invention provides a polymer-conjugatecomprising the structure:POLY-L_(0,1)-C(O)G-Y—S—S-A  (VII)where “A” indicates an active agent, and “S-A” indicates the residue ofan active agent having a thiol group.

In one embodiment of this aspect of the invention, the active agent isselected from the group consisting of proteins, peptides, and smallmolecules.

Also provided herein is a method for delivering a bioactive agent to asubject in need thereof by administering a polymer-conjugate of theinvention.

These and other objects and features of the invention will become morefully apparent when read in conjunction with the following detaileddescription.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The following terms as used herein have the meanings indicated. As usedin the specification, and in the appended claims, the singular forms“a”, “an”, “the”, include plural referents unless the context clearlydictates otherwise.

“Thiol selective derivative”, in the context of a polymer of theinvention, means a polymer having at least one terminus that is athiol-reactive group. Preferentially, a thiol-selective group is onethat reacts preferentially with thiol groups. A thiol-selective polymerof the invention will preferably be fairly selective for thiol groupsunder certain reaction conditions. Exemplary thiol-selective groupsinclude maleimide, vinyl sulfone, orthopyridyl disulfide, iodoacetamide,thiol (—SH), thiolate (—S⁻), or protected thiol, that is to say, a thiolgroup in its protected form. Typical thiol protecting groups includethioether, thioester, or disulfide. Exemplary protecting groups forthiols can be found in Greene, T., and Wuts, Peter G. M., “PROTECTIVEGROUPS IN ORGANIC SYNTHESIS, Chapter 6, 3^(rd) Edition, John Wiley andSons, Inc., New York, 1999 (p. 454-493).

“Activated carboxylic acid” or “activated carboxylic acid derivative”means a functional derivative of a carboxylic acid that is more reactivethan the parent carboxylic acid, in particular, with respect tonucleophilic acyl substitution. Activated carboxylic acids include butare not limited to acid halides (such as acid chlorides), anhydrides,amides and esters.

“PEG” or “poly(ethylene glycol)” as used herein, is meant to encompassany water-soluble poly(ethylene oxide). Typically, PEGs for use in thepresent invention will comprise one of the two following structures:“—(CH₂CH₂O)_(n)—” or “—(CH₂CH₂O)_(n-1)CH₂CH₂—,” depending upon whetheror not the terminal oxygen(s) has been displaced, e.g., during asynthetic transformation. The variable (n) ranges from 3 to 4000, andthe terminal groups and architecture of the overall PEG may vary. WhenPEG further comprises a linker moiety (to be described in greater detailbelow), the atoms comprising the linker, when covalently attached to aPEG segment, do not result in formation of (i) an oxygen-oxygen bond(—O—O—, a peroxide linkage), or (ii) a nitrogen-oxygen bond (N—O, O—N).“PEG” means a polymer that contains a majority, that is to say, greaterthan 50%, of subunits that are —CH₂CH₂O—. PEGs for use in the inventioninclude PEGs having a variety of molecular weights, structures orgeometries (e.g., branched, linear, forked PEGs, dendritic, and thelike), to be described in greater detail below.

“PEG diol”, also known as alpha-, omega-dihydroxylpoly(ethylene glycol),can be represented in brief form as HO-PEG-OH, where PEG is as definedabove.

“Water-soluble”, in the context of a polymer of the invention or a“water-soluble polymer segment” is any segment or polymer that issoluble in water at room temperature. Typically, a water-soluble polymeror segment will transmit at least about 75%, more preferably at leastabout 95% of light, transmitted by the same solution after filtering. Ona weight basis, a water-soluble polymer or segment thereof willpreferably be at least about 35% (by weight) soluble in water, morepreferably at least about 50% (by weight) soluble in water, still morepreferably about 70% (by weight) soluble in water, and still morepreferably about 85% (by weight) soluble in water. It is most preferred,however, that the water-soluble polymer or segment is about 95% (byweight) soluble in water or completely soluble in water.

An “end-capping” or “end-capped” group is an inert or non-reactive grouppresent on a terminus of a polymer such as PEG. An end-capping group isone that does not readily undergo chemical transformation under typicalsynthetic reaction conditions. An end capping group is generally analkoxy group, —OR, where R is an organic radical comprised of 1-20carbons and is preferably lower alkyl (e.g., methyl, ethyl) or benzyl.“R” may be saturated or unsaturated, and includes aryl, heteroaryl,cyclo, heterocyclo, and substituted forms of any of the foregoing. Forinstance, an end capped PEG will typically comprise the structure“RO—(CH₂CH₂O)_(n)CH₂CH₂—”, where R is as defined above. Alternatively,the end-capping group can also advantageously comprise a detectablelabel. When the polymer has an end-capping group comprising a detectablelabel, the amount or location of the polymer and/or the moiety (e.g.,active agent) to which the polymer is coupled, can be determined byusing a suitable detector. Such labels include, without limitation,fluorescers, chemiluminescers, moieties used in enzyme labeling,colorimetric (e.g., dyes), metal ions, radioactive moieties, and thelike. The end-capping group can also advantageously comprise aphospholipid. When the polymer has an end-capping group such as aphospholipid, unique properties (such as the ability to form organizedstructures with similarly end-capped polymers) are imparted to thepolymer. Exemplary phospholipids include, without limitation, thoseselected from the class of phospholipids called phosphatidylcholines.Specific phospholipids include, without limitation, those selected fromthe group consisting of dilauroylphosphatidylcholine,dioleylphosphatidylcholine, dipalmitoylphosphatidylcholine,disteroylphosphatidylcholine, behenoylphosphatidylcholine,arachidoylphosphatidylcholine, and lecithin.

“Non-naturally occurring” with respect to a polymer of the inventionmeans a polymer that in its entirety is not found in nature. Anon-naturally occurring polymer of the invention may however contain oneor more subunits or segments of subunits that are naturally occurring,so long as the overall polymer structure is not found in nature.

“Molecular mass” in the context of a water-soluble polymer of theinvention such as PEG, refers to the nominal average molecular mass of apolymer, typically determined by size exclusion chromatography, lightscattering techniques, or intrinsic velocity determination in1,2,4-trichlorobenzene. The polymers of the invention are typicallypolydisperse, possessing low polydispersity values of less than about1.20, and more preferably less than 1.10.

The term “reactive” or “activated” refers to a functional group thatreacts readily or at a practical rate under conventional conditions oforganic synthesis. This is in contrast to those groups that either donot react or require strong catalysts or impractical reaction conditionsin order to react (i.e., a “nonreactive” or “inert” group).

“Not readily reactive” or “inert” with reference to a functional grouppresent on a molecule in a reaction mixture, indicates that the groupremains largely intact under conditions effective to produce a desiredreaction in the reaction mixture.

A “protecting group” is a moiety that prevents or blocks reaction of aparticular chemically reactive functional group in a molecule undercertain reaction conditions. The protecting group will vary dependingupon the type of chemically reactive group being protected as well asthe reaction conditions to be employed and the presence of additionalreactive or protecting groups in the molecule. Functional groups whichmay be protected include, by way of example, carboxylic acid groups,amino groups, hydroxyl groups, thiol groups, carbonyl groups and thelike. Representative protecting groups for carboxylic acids includeesters (such as a p-methoxybenzyl ester), amides and hydrazides; foramino groups, carbamates (such as tert-butoxycarbonyl) and amides; forhydroxyl groups, ethers and esters; for thiol groups, thioethers andthioesters; for carbonyl groups, acetals and ketals; and the like. Suchprotecting groups are well-known to those skilled in the art and aredescribed, for example, in T. W. Greene and G. M. Wuts, ProtectingGroups in Organic Synthesis, Third Edition, Wiley, New York, 1999, andreferences cited therein.

A functional group in “protected form” refers to a functional groupbearing a protecting group. As used herein, the term “functional group”or any synonym thereof is meant to encompass protected forms thereof.

The term “linker” is used herein to refer to an atom or a collection ofatoms optionally used to link interconnecting moieties, such as apolymer segment and an electrophile. The linkers of the invention aregenerally hydrolytically stable.

A “physiologically cleavable” or “hydrolyzable” or “degradable” bond isa relatively weak bond that reacts with water (i.e., is hydrolyzed)under physiological conditions. The tendency of a bond to hydrolyze inwater will depend not only on the general type of linkage connecting twocentral atoms but also on the substituents attached to these centralatoms. Appropriate hydrolytically unstable or weak linkages include butare not limited to carboxylate ester, phosphate ester, anhydrides,acetals, ketals, acyloxyalkyl ether, imines, orthoesters, peptides andoligonucleotides, thioesters, thiolesters, and carbonates.

An “enzymatically degradable linkage” means a linkage that is subject todegradation by one or more enzymes.

A “hydrolytically stable” linkage or linker, for the purposes of thepresent invention, and in particular in reference to the polymers of theinvention, refers to an atom or to a collection of atoms, that ishydrolytically stable under normal physiological conditions. That is tosay, a hydrolytically stable linkage does not undergo hydrolysis underphysiological conditions to any appreciable extent over an extendedperiod of time. Examples of hydrolytically stable linkages include butare not limited to the following: carbon-carbon bonds (e.g., inaliphatic chains), ethers, amides, urethanes, amines, and the like.Hydrolysis rates of representative chemical bonds can be found in moststandard chemistry textbooks.

“Branched” in reference to the geometry or overall structure of apolymer refers to polymer having 2 or more polymer “arms”. A branchedpolymer may possess 2 polymer arms, 3 polymer arms, 4 polymer arms, 6polymer arms, 8 polymer arms or more. One particular type of highlybranched polymer is a dendritic polymer or dendrimer, that for thepurposes of the invention, is considered to possess a structure distinctfrom that of a branched polymer.

“Branch point” refers to a bifurcation point comprising one or moreatoms at which a polymer splits or branches from a linear structure intoone or more additional polymer arms.

A “dendrimer” is a globular, size monodisperse polymer in which allbonds emerge radially from a central focal point or core with a regularbranching pattern and with repeat units that each contribute a branchpoint. Dendrimers exhibit certain dendritic state properties such ascore encapsulation, making them unique from other types of polymers.

“Substantially” or “essentially” means nearly totally or completely, forinstance, 95% or greater of some given quantity.

An “alkyl” or “alkylene” group, depending upon its position in amolecule and the number of points of attachment of the group to atomsother than hydrogen, refers to a hydrocarbon chain or moiety, typicallyranging from about 1 to 20 atoms in length. Such hydrocarbon chains arepreferably but not necessarily saturated unless so indicated and may bebranched or straight chain, although typically straight chain ispreferred. Exemplary alkyl groups include methyl, ethyl, propyl, butyl,pentyl, 1-methylbutyl, 1-ethylpropyl, 3-methylpentyl, and the like.

“Lower alkyl” or “lower alkylene” refers to an alkyl or alkylene groupas defined above containing from 1 to 6 carbon atoms, and may bestraight chain or branched, as exemplified by methyl, ethyl, n-butyl,i-butyl, t-butyl.

“Cycloalkyl” or “cycloalkylene”, depending upon its position in amolecule and the number of points of attachment to atoms other thanhydrogen, refers to a saturated or unsaturated cyclic hydrocarbon chain,including polycyclics such as bridged, fused, or spiro cyclic compounds,preferably made up of 3 to about 12 carbon atoms, more preferably 3 toabout 8.

“Lower cycloalkyl” or “lower cycloalkylene” refers to a cycloalkyl groupcontaining from 1 to 6 carbon atoms.

“Alicyclic” refers to any aliphatic compound that contains a ring ofcarbon atoms. An alicyclic group is one that contains a “cycloalkyl” or“cycloalkylene” group as defined above that is substituted with one ormore alkyl or alkylenes.

“Non-interfering substituents” are those groups that, when present in amolecule, are typically non-reactive with other functional groupscontained within the molecule.

The term “substituted” as in, for example, “substituted alkyl,” refersto a moiety (e.g., an alkyl group) substituted with one or morenon-interfering substituents, such as, but not limited to: C₃-C₈cycloalkyl, e.g., cyclopropyl, cyclobutyl, and the like; halo, e.g.,fluoro, chloro, bromo, and iodo; cyano; alkoxy, lower phenyl;substituted phenyl; and the like. For substitutions on a phenyl ring,the substituents may be in any orientation (i.e., ortho, meta, or para).

“Alkoxy” refers to an —O—R group, wherein R is alkyl or substitutedalkyl, preferably C₁-C₂₀ alkyl (e.g., methoxy, ethoxy, propyloxy,benzyl, etc.), preferably C₁-C₇.

As used herein, “alkenyl” refers to a branched or unbranched hydrocarbongroup of 1 to 15 atoms in length, containing at least one double bond,such as ethenyl, n-propenyl, isopropenyl, n-butenyl, isobutenyl,octenyl, decenyl, tetradecenyl, and the like.

The term “alkynyl” as used herein refers to a branched or unbranchedhydrocarbon group of 2 to 15 atoms in length, containing at least onetriple bond, ethynyl, n-propynyl, isopropynyl, n-butynyl, isobutynyl,octynyl, decynyl, and so forth.

“Aryl” means one or more aromatic rings, each of 5 or 6 core carbonatoms. Aryl includes multiple aryl rings that may be fused, as innaphthyl or unfused, as in biphenyl. Aryl rings may also be fused orunfused with one or more cyclic hydrocarbon, heteroaryl, or heterocyclicrings. As used herein, “aryl” includes heteroaryl.

“Heteroaryl” is an aryl group containing from one to four heteroatoms,preferably N, O, or S, or a combination thereof. Heteroaryl rings mayalso be fused with one or more cyclic hydrocarbon, heterocyclic, aryl,or heteroaryl rings.

“Heterocycle” or “heterocyclic” means one or more rings of 5-12 atoms,preferably 5-7 atoms, with or without unsaturation or aromatic characterand having at least one ring atom which is not a carbon. Preferredheteroatoms include sulfur, oxygen, and nitrogen.

“Substituted heteroaryl” is heteroaryl having one or morenon-interfering groups as substituents.

“Substituted heterocycle” is a heterocycle having one or more sidechains formed from non-interfering substituents.

“Electrophile” refers to an ion, atom, or collection of atoms that maybe ionic, having an electrophilic center, i.e., a center that iselectron seeking, capable of reacting with a nucleophile.

“Nucleophile” refers to an ion or atom or collection of atoms that maybe ionic, having a nucleophilic center, i.e., a center that is seekingan electrophilic center, and capable of reacting with an electrophile.

“Active agent” as used herein includes any agent, drug, compound,composition of matter or mixture which provides some pharmacologic,often beneficial, effect that can be demonstrated in-vivo or in vitro.This includes foods, food supplements, nutrients, nutriceuticals, drugs,vaccines, antibodies, vitamins, and other beneficial agents. As usedherein, these terms further include any physiologically orpharmacologically active substance that produces a localized or systemiceffect in a patient.

“Pharmaceutically acceptable excipient” or “pharmaceutically acceptablecarrier” refers to an excipient that can be included in the compositionsof the invention and that causes no significant adverse toxicologicaleffects to the patient.

“Pharmacologically effective amount,” “physiologically effectiveamount,” and “therapeutically effective amount” are used interchangeablyherein to mean the amount of a PEG-active agent conjugate present in apharmaceutical preparation that is needed to provide a desired level ofactive agent and/or conjugate in the bloodstream or in the targettissue. The precise amount will depend upon numerous factors, e.g., theparticular active agent, the components and physical characteristics ofpharmaceutical preparation, intended patient population, patientconsiderations, and the like, and can readily be determined by oneskilled in the art, based upon the information provided herein andavailable in the relevant literature.

“Multi-functional” in the context of a polymer of the invention means apolymer backbone having 3 or more functional groups contained therein,where the functional groups may be the same or different, and aretypically present on the polymer termini. Multi-functional polymers ofthe invention will typically contain from about 3-100 functional groups,or from 3-50 functional groups, or from 3-25 functional groups, or from3-15 functional groups, or from 3 to 10 functional groups, or willcontain 3, 4, 5, 6, 7, 8, 9 or 10 functional groups within the polymerbackbone.

A “difunctional” polymer means a polymer having two functional groupscontained therein, typically at the polymer termini. When the functionalgroups are the same, the polymer is said to be homodifunctional. Whenthe functional groups are different, the polymer is said to beheterobifunctional.

A basic or acidic reactant described herein includes neutral, charged,and any corresponding salt forms thereof.

“Polyolefinic alcohol” refers to a polymer comprising an olefin polymerbackbone, such as polyethylene, having multiple pendant hydroxyl groupsattached to the polymer backbone. An exemplary polyolefinic alcohol ispolyvinyl alcohol.

As used herein, “non-peptidic” refers to a polymer backbonesubstantially free of peptide linkages. However, the polymer may includea minor number of peptide linkages spaced along the repeat monomersubunits, such as, for example, no more than about 1 peptide linkage perabout 50 monomer units.

The term “patient,” refers to a living organism suffering from or proneto a condition that can be prevented or treated by administration of apolymer of the invention, typically but not necessarily in the form of apolymer-active agent conjugate, and includes both humans and animals.

“Optional” or “optionally” means that the subsequently describedcircumstance may or may not occur, so that the description includesinstances where the circumstance occurs and instances where it does not.

By “residue” is meant the portion of a molecule remaining after reactionwith one or more molecules. For example, a biologically active moleculeresidue in a polymer conjugate of the invention typically corresponds tothe portion of the biologically active molecule up to but excluding thecovalent linkage resulting from reaction of a reactive group on thebiologically active molecule with a reactive group on a polymer reagent.The term “conjugate” is intended to refer to the entity formed as aresult of covalent attachment of a molecule, e.g., a biologically activemolecule or any reactive surface, to a reactive polymer molecule,preferably a reactive poly(ethylene glycol).

“Cysteamine” refers to 2-aminoethanethiol, or H₂N—(CH₂)₂—SH.

“Cystamine” refers to 2,2′-dithiobis(ethylamine) or (H₂N—(CH₂)₂—S—)₂.

Method for Preparing Thiol-Selective Derivatives of Water SolublePolymers

Overview of the Method

The present invention provides a method for preparing water-solublepolymer derivatives suitable for reaction with thiol groups on proteinsor on other active agents. In the method, a water soluble polymersegment having at least one reactive electrophilic terminus is reactedwith a bifunctional reactant molecule (that is to say, a reactantmolecule possessing at least two functional groups as described below)that contains both a nucleophile (for reaction with the electrophilicterminus of the polymer) and a thiol-selective moiety. Representativethiol selective moieties include thiol, protected thiol, disulfide,maleimide, organomercurials, alpha-haloacetyl compounds such asiodoacetamide, vinyl sulfones, aryl halides, diazoacetates, andorthopyridyl disulfide. The reaction is carried out under conditionseffective to promote reaction between the electrophilic terminus of thepolymer and the nucleophilic group of the reactant molecule, to form acovalent attachment between the polymer and the reactant molecule. Thereaction results in the formation of an activated polymer having aterminus that is selective for reaction with a thiol (e.g., thiol,protected thiol, disulfide, maleimide, vinyl sulfone, iodoacetamide ororthopyridyl disulfide), depending upon the particular reactant moleculeemployed. A generalized reaction scheme is presented below.

1. Generalized Reaction Scheme:

2. Exemplary Specific Embodiment

where R.A. is a reducing agent, and Z, POLY, L, Y, X, NU are as definedherein.

where A is an active agent.

In its most simplified form, the method can provide activatedpolyethylene glycol derivatives having a thiol-selective terminus (e.g.,a thiol, protected thiol, or maleimide) in one reaction step. Ininstances in which the reactant molecule possesses a nucleophile thatcompetes with the thiol-selective moiety for reaction with theelectrophilic terminus of the polymer, for example when the nucleophileis an amino group and the sulfur containing moiety is a thiol or athiolate, such as in the exemplary reactant, cysteamine, protection ofthe thiol group on the reactant molecule may be necessary to preventreaction of the polymer at the thiol-center of the reactant.Alternatively, if the rates of reaction of the two competing moietiesare significantly different, the reaction may be carried out underconditions where the nucleophilic center of the reactant molecule isselectively or preferentially reacted with the electrophile of thepolymer. Undesired reaction products resulting from the reaction betweenthe thiol or thiolate and the given electrophile can then be removed byadditional purification/separation steps.

A detailed description regarding suitable electrophilically activatedpolymers and molecular reactants is provided in the sections thatfollow.

In one preferred embodiment, the reactant molecule is a symmetricaldisulfide having two identical nucleophilic groups for reaction with theelectrophilic group of the polymer. This approach is advantageousbecause no competition exists between potentially different nucleophilesin the reactant molecule. Thus, under suitable reaction conditions(e.g., when an at least a two fold molar excess of electrophilicallyactivated polymer is employed-sufficient to react with all of thenucleophile groups in the symmetrical disulfide), only one activatedpolymer product is formed.

Exemplary symmetrical reactant molecules will possess a centraldisulfide (—S—S—) bond where the sulfur atoms are each connected toidentical Y groups such as alkylene, substituted alkylene,cycloalkylene, substituted cycloalkylene, aryl, or substituted arylgroup possessing a nucleophile (NU) such as an amino, hydroxy, thiol,imino, thioester or the like covalently attached thereto, “(NU-Y—S)₂”.One such preferred symmetrical reactant molecule is cystamine. Reactionof an electrophilically activated polymer with a symmetrical disulfidereagent such as cystamine results in formation of a symmetricaldisulfide polymer having identical polymer segments extending from eachof the sulfur atoms of a central disulfide linkage. Illustrativereactions carried out with different electrophilically activatedpolymers and the reactant, cystamine, are provided in the Examples(Examples 1-3). The electrophilically-activated polymer is typicallyreacted with a bi-functional reactant molecule under very mild reactionconditions (e.g., room temperature), offering another advantage of thisapproach. Moreover, typical yields are greater than 70%, preferablygreater than 80%, more preferably greater than 90%, and often evengreater than 95%.

Due to the symmetry of the resulting disulfide polymer, cleavage byaction of a reducing agent such as dithiothreitol results in formationof two moles of the corresponding thiol-selective polymer derivative(“POLY-S”).

Preferred polymers (POLY-E's) for use in the present invention includemethoxy-PEG propanoic acid and methoxy-PEG butanoic acid and activatedforms thereof. Particularly preferred polymer derivatives having a thiolselective terminus are provided in structures (11), (13), and (18)described herein.

In another preferred embodiment, the polymer derivatives of theinvention are prepared from polymer acid or polymer acid-equivalentstarting materials, where the polymer acid is purified prior to thereaction with the nucleophile. A polymer acid or polymer acid equivalentis a water soluble polymer of the invention having at least onefunctional group or terminus that is a carboxylic acid or a carboxylicacid-equivalent such as an activated derivative of a carboxylic acid.The use or preparation of a polymer acid provides an additionaladvantage in that it allows for the ready removal of PEG diol or PEGdiol-derived impurities that may be present in the polymer startingmaterial, depending upon its source.

Often, polyethylene glycol starting materials, such as electrophilicallyactivated PEG, as used in many embodiments of the invention, containdetectable amounts of PEG diol impurity, often ranging from 0.5% to over30% by weight. Any amount of diol impurity can be a problem, since thediol (and its reaction products) can be extremely difficult toremove/separate. Additionally, due to its reactivity, PEG-diol (and moreparticularly its conversion products) can react with a bioactive agentduring a coupling reaction, resulting in the formation of a mixture ofconjugate products. The resulting mixture of conjugates can be difficultto analyze, i.e., to determine the extent of diol-derived impuritiespresent. Moreover, separation of the desired conjugate product from thediol-derived conjugate products can be extremely difficult, and in someinstances, may be impossible to achieve. In particular, high molecularweight water soluble polymers such as methoxy-PEG-OH (e.g., having amolecular weight of greater than about 30,000 daltons) can contain up to30 percent by weight or more diol, depending upon the source and/or themethod of making the PEG starting material. As discussed above, suchdiol and diol-derived impurities can be especially problematic whencarried through a series of synthetic transformations and/or aconjugation reaction. The use of a polymer acid, such as in method ofthe invention, allows for the purification, e.g., by chromatography, ofthe POLY-E starting material (or its equivalent precursor) and theultimate formation of a thiol-selective polymer formulation that isessentially free of reactive PEG-diol or reactive PEG-diol derivedimpurities.

The applicants have recognized that separation of PEG-diol-relatedimpurities at the front end of a reaction or series of reactions leadingto the formation of a reactive polymer derivative or eventually apolymer conjugate is advantageous, since separation/purification at thisstage is more readily accomplished when compared to the separation ofvarious polymer conjugate species.

Alternatively, as an approach for removing or rendering inert mPEG-diolderived impurities, the water-soluble polymer provided in step (i),POLY-E, may be prepared from a diol-free PEG-OH prepared frombenzyloxy-PEG as described in co-owned U.S. Pat. No. 6,448,369. In thisapproach, the benzyloxy-PEG-OH starting material is preparedpreferentially by polymerization of ethylene oxide onto the benzyloxideion, Bz—O⁻, resulting in high purity monofunctional benzyloxy PEGscontaining PEG-diol. After converting all PEG-OH groups to inert methylethers and removing benzyloxy groups in subsequent steps, the methodprovides pure, diol-free methoxy-PEG-OH. In utilizing this method,PEG-diol is converted to its non-reactive ether form, rendering it aninert component of the resulting composition.

In sum, the method provided herein (i) avoids multiple cumbersomereaction steps, (ii) does not necessarily require multipleprotecting/deprotecting steps, (iii) is carried out under mildconditions such that the polymer segment is not particularly susceptibleto damage, (iv) results in a high yields of product, typically greaterthan 90%, and (v) provides a new class of polymer derivatives having athiol-selective terminus. The overall synthetic methodology, reagents,polymer derivatives, compositions, and conjugates of the invention willnow be described more fully below.

Polymer Reactants, POLY-L0,1-E

the Polymer Segment, POLY

The following describes the polymer segment designated herein as POLY,applicable to the electrophilically activated polymers of the method, aswell as the thiol-selective polymers of the invention.Electrophilically-activated polymer derivatives useful in the presentinvention generally comprise at least one electrophile coupled to awater soluble polymer segment. The electrophile can either be covalentlybonded directly to the polymer segment, or alternatively can be coupledto the polymer backbone via a linking group, L.

Representative POLYs include poly(alkylene glycols) such aspoly(ethylene glycol), poly(propylene glycol) (“PPG”), copolymers ofethylene glycol and propylene glycol, poly(olefinic alcohol),poly(vinylpyrrolidone), poly(hydroxyalkylmethacrylamide),poly(hydroxyalkylmethacrylate), poly(saccharides), poly(α-hydroxy acid),poly(vinyl alcohol), polyphosphazene, polyoxazoline, andpoly(N-acryloylmorpholine). POLY can be a homopolymer, an alternatingcopolymer, a random copolymer, a block copolymer, an alternatingtripolymer, a random tripolymer, or a block tripolymer of any of theabove. The water-soluble polymer segment is preferably, although notnecessarily, a poly(ethylene glycol) “PEG” or a derivative thereof.

The polymer segment can have any of a number of different geometries,for example, POLY can be linear, branched, or forked. Most typically,POLY is linear or is branched, for example, having 2 polymer arms.Although much of the discussion herein is focused upon PEG as anillustrative POLY, the discussion and structures presented herein can bereadily extended to encompass any of the water-soluble polymer segmentsdescribed above.

Any water-soluble polymer having at least one electrophilicallyactivated terminus can be used to prepare a thiol-selective polymer inaccordance with the method of the invention. Although water-solublepolymers bearing only a single reactive electrophilically activatedterminus are typically used and illustrated herein, polymers bearingtwo, three, four, five, six, seven, eight, nine, ten, eleven, twelve ormore reactive termini suitable for conversion to a thiol selectivepolymer as set forth herein can be used. Advantageously, as the numberof hydroxyl or other reactive moieties on the water-polymer segmentincreases, the number of available sites for introducing anelectrophilic group increases. Non-limiting examples of the upper limitof the number of hydroxyl and/or electrophilic moieties associated withthe water-soluble polymer segment include from about 1 to about 500,from 1 to about 100, from about 1 to about 80, from about 1 to about 40,from about 1 to about 20, and from about 1 to about 10.

In turning now to the preferred POLY, PEG encompasses poly(ethyleneglycol) in any of its linear, branched or multi-arm forms, includingend-capped PEG, forked PEG, branched PEG, pendant PEG, and PEGcontaining one or more degradable linkage separating the monomersubunits, to be more fully described below.

A PEG polymer segment comprises the following: —(CH₂CH₂O)_(n)—CH₂CH₂—,where (n) typically ranges from about 3 to about 4,000, or from about 3to about 3,000, or more preferably from about 20 to about 1,000.

POLY can be end-capped, where PEG is terminally capped with an inertend-capping group. Preferred end-capped PEGs are those having as anend-capping moiety such as alkoxy, substituted alkoxy, alkenyloxy,substituted alkenyloxy, alkynyloxy, substituted alkynyloxy, aryloxy,substituted aryloxy. Preferred end-capping groups are C₁-C₂₀ alkoxy suchas methoxy, ethoxy, and benzyloxy. The end-capping group can alsoadvantageously comprise a phospholipid. Exemplary phospholipids includephosphatidylcholines, such as dilauroylphosphatidylcholine,dioleylphosphatidylcholine, dipalmitoylphosphatidylcholine,disteroylphosphatidylcholine, behenoylphosphatidylcholine,arachidoylphosphatidylcholine, and lecithin.

Referring now to any of the structures comprising a polymer segment,POLY, POLY may correspond or comprise the following:“Z—(CH₂CH₂O)_(n)—” or “Z—(CH₂CH₂O)_(n)—CH₂CH₂—”,where n ranges from about 3 to about 4000, or from about 10 to about4000, and Z is or includes a functional group, which may be a reactivegroup or an end-capping group. Examples of Z include hydroxy, amino,ester, carbonate, aldehyde, acetal, aldehyde hydrate, ketone, ketal,ketone hydrate, alkenyl, acrylate, methacrylate, acrylamide, sulfone,thiol, carboxylic acid, isocyanate, isothiocyanate, hydrazide, urea,maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide,alkoxy, benzyloxy, silane, lipid, phospholipid, biotin, and fluorescein,including activated and protected forms thereof where applicable.Preferred are functional groups such as N-hydroxysuccinimidyl ester,1-hydroxybenzotriazolyl carbonate, amine, vinylsulfone, maleimide,N-succinimidyl carbonate, hydrazide, succinimidyl propionate,succinimidyl butanoate, succinimidyl succinate, succinimidyl ester,glycidyl ether, oxycarbonylimidazole, p-nitrophenyl carbonate, aldehyde,orthopyridyl-disulfide, and acrylol.

The polymer reactant (and corresponding product) may possess adumbbell-like or bifunctional linear structure, e.g., in which twoelectrophiles are interconnected by a central POLY, e.g., PEG. Morespecifically, such POLY may be represented by the structure E1-PEG-E2,where E1 and E2 are independently selected electrophiles as describedherein. Preferably, E1 and E2 are the same. Exemplary PEGs falling intothis classification are provided in Example 3, e.g., (15) and (16).Additional examples are provided in U.S. Pat. No. 5,900,461, the contentof which is expressly incorporated herein by reference. In a preferredembodiment, particularly in regard to the thiol-selective polymers ofthe invention, or their precursors, the functional group, Z, maycorrespond to “L_(0,1)-X—Y—S” to provide a homo-bifunctionalthiol-selective polymer having identical groups on either side of thepolymer segment, e.g.,S—Y—X-L_(0,1)-POLY-L_(0,1)-X-Y—S,   VIII.

These and other functional groups, Z, are described in the followingreferences, all of which are incorporated by reference herein:N-succinimidyl carbonate (see e.g., U.S. Pat. Nos. 5,281,698,5,468,478), amine (see, e.g., Buckmann et al. Makromol. Chem. 182:1379(1981), Zalipsky et al. Eur. Polym. J. 19:1177 (1983)), hydrazide (See,e.g., Andresz et al. Makromol. Chem. 179:301 (1978)), succinimidylpropionate and succinimidyl butanoate (see, e.g., Olson et al. inPoly(ethylene glycol) Chemistry & Biological Applications, pp 170-181,Harris & Zalipsky Eds., ACS, Washington, D.C., 1997; see also U.S. Pat.No. 5,672,662), succinimidyl succinate (See, e.g., Abuchowski et al.Cancer Biochem. Biophys. 7:175 (1984) and Joppich et al., Makromol.Chem. 180:1381 (1979), succinimidyl ester (see, e.g., U.S. Pat. No.4,670,417), benzotriazole carbonate (see, e.g., U.S. Pat. No.5,650,234), glycidyl ether (see, e.g., Pitha et al. Eur. J. Biochem.94:11 (1979), Elling et al., Biotech. Appl. Biochem. 13:354 (1991),oxycarbonylimidazole (see, e.g., Beauchamp, et al., Anal. Biochem.131:25 (1983), Tondelli et al. J. Controlled Release 1:251 (1985)),p-nitrophenyl carbonate (see, e.g., Veronese, et al., Appl. Biochem.Biotech., 11:141 (1985); and Sartore et al., Appl. Biochem. Biotech.,27:45 (1991)), aldehyde (see, e.g., Harris et al. J. Polym. Sci. Chem.Ed. 22:341 (1984), U.S. Pat. No. 5,824,784, U.S. Pat. No. 5,252,714),maleimide (see, e.g., Goodson et al. Bio/Technology 8:343 (1990), Romaniet al. in Chemistry of Peptides and Proteins 2:29 (1984)), and Kogan,Synthetic Comm. 22:2417 (1992)), orthopyridyl-disulfide (see, e.g.,Woghiren, et al. Bioconj. Chem. 4:314 (1993)), acrylol (see, e.g.,Sawhney et al., Macromolecules, 26:581 (1993)), vinylsulfone (see, e.g.,U.S. Pat. No. 5,900,461).

Again, the POLY types described are meant to encompass linear polymersegments, and also branched or forked polymer segments. In an instancewhere the polymer segment is branched, the POLY structure may, forexample, correspond to the polymer arms forming part of the overall POLYstructure. Alternatively, in an instance where POLY possesses a forkedstructure, POLY may, for example, correspond to the linear portion ofthe polymer segment prior to the branch point.

POLY also encompasses branched PEG molecules having 2 arms, 3 arms, 4arms, 5 arms, 6 arms, 7 arms, 8 arms or more. Branched polymers used toprepare the thiol-selective polymers of the invention may possessanywhere from 2 to 300 or so reactive termini. Preferred are branchedpolymer segments having 2 or 3 polymer arms. An illustrative branchedPOLY, as described in U.S. Pat. No. 5,932,462, corresponds to thestructure:

In this representation, R″ is a nonreactive moiety, such as H, methyl ora PEG, and P and Q are non-reactive linkages. In a preferred embodiment,the branched PEG polymer segment is methoxy poly(ethylene glycol)disubstituted lysine.

In the above particular branched configuration, the branched polymersegment possesses a single reactive site extending from the “C” branchpoint for positioning of the reactive electrophilic group of the polymerreactant or the X group of the thiol-selective polymer product. BranchedPEGs such as these for use in the present invention will typically havefewer than 4 PEG arms, and more preferably, will have 2 or 3 PEG arms.Such branched PEGs offer the advantage of having a single reactive site,coupled with a larger, more dense polymer cloud than their linear PEGcounterparts. Illustrative branched polymers bearing electrophilicgroups such as these are commercially available from Nektar (Huntsville,Ala.), and include mPEG2-N-hydroxysuccinimide.

An illustrative branched polymer reactant and correspondingthiol-selective polymer of the invention, respectively, have thestructures shown below:

where X, Y, and S are as described herein. In structure IX-B, X maycorrespond to —C(O)-G, where G is —NH.

Branched polymers for use in preparing a polymer of the inventionadditionally include those represented more generally by the formulaR(POLY)_(n), where R is a central or core molecule from which extends 2or more POLY arms such as PEG. The variable n represents the number ofPOLY arms, where each of the polymer arms can independently beend-capped or alternatively, possess a reactive functional group at itsterminus. Typically, at least one polymer arm possesses a terminalfunctional group. In such multi-armed embodiments of the invention, eachPEG arm typically possesses an electrophile at its terminus (or thecorresponding reaction product between the electrophile and thenucleophile as previously described). Branched PEGs such as thoserepresented generally by the formula, R(PEG)_(n), above possess 2polymer arms to about 300 polymer arms (i.e., n ranges from 2 to about300). Preferably, such branched PEGs possess from 2 to about 25 polymerarms, more preferably from 2 to about 20 polymer arms, and even morepreferably from 2 to about 15 polymer arms or fewer. Most preferred aremulti-armed polymers having 3, 4, 5, 6, 7 or 8 arms.

Preferred core molecules in branched PEGs as described above arepolyols. Such polyols include aliphatic polyols having from 1 to 10carbon atoms and from 1 to 10 hydroxyl groups, including ethyleneglycol, alkane diols, alkyl glycols, alkylidene alkyl diols, alkylcycloalkane diols, 1,5-decalindiol,4,8-bis(hydroxymethyl)tricyclodecane, cycloalkylidene diols,dihydroxyalkanes, trihydroxyalkanes, and the like. Cycloaliphaticpolyols may also be employed, including straight chained or closed-ringsugars and sugar alcohols, such as mannitol, sorbitol, inositol,xylitol, quebrachitol, threitol, arabitol, erythritol, adonitol,ducitol, facose, ribose, arabinose, xylose, lyxose, rhamnose, galactose,glucose, fructose, sorbose, mannose, pyranose, altrose, talose,tagitose, pyranosides, sucrose, lactose, maltose, and the like.Additional aliphatic polyols include derivatives of glyceraldehyde,glucose, ribose, mannose, galactose, and related stereoisomers. Othercore polyols that may be used include crown ether, cyclodextrins,dextrins and other carbohydrates such as starches and amylose. Preferredpolyols include glycerol, pentaerythritol, sorbitol, andtrimethylolpropane.

A representative multi-arm structure corresponding to a thiol-selectivepolymer of the invention is shown below, where n preferably ranges fromabout 3 to about 8.

Additional multi-arm polymers for use in preparing a thiol-selectivepolymer of the invention include multi-arm PEGs available from Nektar(Huntsville, Ala.). Preferred multi-armed electrophilically activatedpolymers for use in the method of the invention correspond to thefollowing structure, where E represents an electrophilic group,

PEG is —(CH₂CH₂O)_(n)CH₂CH₂—, and m is selected from the groupconsisting of 3, 4, 5, 6, 7, and 8. Of course, the correspondingthiol-selective polymer product possesses the structure shown above withthe exception that the electrophile, E, is replaced by “—X—Y—S” (XI-B).

Alternatively, the polymer segment may possess an overall forkedstructure. An example of a forked PEG corresponds to the followinggeneralized structure, where the first structure represents anelectrophilically activated forked PEG and the second structurerepresents a forked thiol-selective polymer product:

where PEG is any of the forms of PEG described herein, A is a linkinggroup, preferably a hydrolytically stable linkage such as oxygen,sulfur, or —C(O)—NH—, F and F′ are hydrolytically stable spacer groupsthat are optionally present, and the other variables corresponding to E,X, Y, and S are as defined above. Both the general and specificdescriptions of possible values for X, Y and S are applicable to thestructure above. Examplary linkers and spacer groups corresponding to A,F and F′ are described in International Application No. PCT/US99/05333,and are useful in forming polymer segments of this type for use in thepresent invention. F and F′ are spacer groups that may be the same ofdifferent. In one particular embodiment of the above, PEG is mPEG, Acorresponds to —C(O)—NH—, and F and F′ are both methylene or —CH₂—. Thistype of polymer segment is useful for reaction with two active agents,where the two active agents are positioned a precise or predetermineddistance apart, depending upon the selection of F and F′.

In any of the representative structures provided herein, one or moredegradable linkages may be contained in the polymer segment to allowgeneration in vivo of a PEG-disulfide linked conjugate having a smallerPEG chain than in the initially administered conjugate. Appropriatephysiologically cleavable linkages include but are not limited to ester,carbonate ester, carbamate, sulfate, phosphate, acyloxyalkyl ether,acetal, and ketal. Such linkages when contained in a given polymersegment will preferably be stable upon storage and upon initialadministration. More particularly, as described generally above, two ormore polymer segments connected by a hydrolyzable linkage may berepresented by the following structure: PEG1-W-PEG2 (where PEG1 and PEG2can be the same or different) and W represents a weak, hydrolyzablelinkage. These polymer structures contain PEG arms or portions of PEGarms that are removable (i.e., cleavable) in-vivo.

Additional representative PEGs having either linear or branchedstructures for use in preparing the conjugates of the invention may bepurchased from Nektar Therapeutics (formerly Shearwater Corporation,Huntsville, Ala.). Illustrative structures are described in Shearwater's2001 catalogue entitled “Polyethylene Glycol and Derivatives forBiomedical Applications”, the contents of which is expresslyincorporated herein by reference.

Generally, the nominal average molecular mass of the water-solublepolymer segment, POLY will vary. The nominal average molecular mass ofPOLY typically falls in one or more of the following ranges: about 100daltons to about 100,000 daltons; from about 500 daltons to about 80,000daltons; from about 1,000 daltons to about 50,000 daltons; from about2,000 daltons to about 25,000 daltons; from about 5,000 daltons to about20,000 daltons. Exemplary nominal average molecular masses for thewater-soluble polymer segment POLY include about 1,000 daltons, about5,000 daltons, about 10,000 daltons, about 15,000 daltons, about 20,000daltons, about 25,000 daltons, about 30,000 daltons, and about 40,000daltons. Low molecular weight POLYs possess molecular masses of about250, 500, 750, 1000, 2000, or 5000 daltons. Exemplary thiol selectivederivatives comprise PEGs having a molecular weight selected from thegroup consisting of 5,000 daltons, 20,000 daltons and 40,000 daltons asprovided in Examples 1-3.

In one particular embodiment of the invention, an activated thiolselective derivative as provided herein possesses a PEG segment havingone of the following nominal average molecular masses: 500, 1000, 2000,3000, 5000, 10,000, 15,000, 20,000, 30,000 and 40,000 daltons.

In terms of the number of subunits, PEGs for use in the invention willtypically comprise a number of (—OCH₂CH₂—) subunits falling within oneor more of the following ranges: 10 to about 4000 subunits, from about20 to about 1000 subunits, from about 25 to about 750 subunits, fromabout 30 to about 500 subunits.

Although any of a number of polymers (POLY) may be utilized, in oneembodiment, the polymer comprises a hydrophilic polymer, that is to say,a polymer containing fewer than about 25 subunits of polypropylene oxideor other similar hydrophobic polymer segments. The polymer may, in analternative embodiment, be absent polypropylene oxide or similarhydrophobic subunits. In one instance, the polymer is preferably not apluronic-type polymer. In yet another particular embodiment, the polymeris preferably not bound to a solid support. In yet another specificinstance, a polymer of the invention is one that may be although is notnecessarily substantially absent fatty acid groups or other lipophilicmoieties.

Polymer Electrophilic Groups (“E”)

A polymer for use in the method contains as least one electrophile orelectrophilic group (-E) suitable for reaction with a nucleophile, suchas that contained in the thiol-selective reactant molecule. Exemplaryelectrophiles include activated esters (e.g. N-hydroxysuccinimidyl (NHS)ester or 1-hydroxybenzotriazolyl ester), active carbonates (e.g.N-hydroxysuccinimidyl carbonate, para-nitrophenylcarbonate, and1-hydroxybenzotriazolyl carbonate), acetal, aldehyde, aldehyde hydrate,active anhydrides such as acid anhydrides, acid halide, aryl halide,ketone, carboxylic acid, isocyanate, isothiocyanate, imidoester, and thelike. Particularly preferred are activated esters such as NHS esters. Insum, a polymer segment having the generalized structure, POLY-L_(0,1)-E,can possess any electrophilic group at at least one terminus, whereexemplary electrophiles include those described above.

Polymer Linkers, L

In most cases, the polymer segment is directly attached to theelectrophile. Alternatively, the polymer segment is attached to theelectrophile via an intervening linker, L. When such a linker isutilized, it is represented generally herein as L₁, meaning that such alinker is present. When such a linker is absent, it is representedherein generally as L₀. Linkers for use in the instant invention aretypically C₁-C₁₀ alkyl or C₁-C₁₀ substituted alkyl. One particularlypreferred linker, e.g., in the instance where the polymer segment isPEG, e.g., —(CH₂CH₂—O)_(n)—CH₂CH₂—, is a methylene group, —CH₂—,although any linear lower alkyl, or branched lower alkyl, or theirsubstituted counterparts may similarly be employed.

Purified Electrophilically Activated Polymers

Particular suitable for use in the present method are electrophilicallyactivated PEG reagents such as chromatographically purified carboxylicacids or their functional equivalents, such as mPEG-succinimidylpropionate, mPEG-succinimidyl butanoate, mPEG-CM-HBA-NHS, mPEG2-NHS, andthe like available from Nektar, Huntsville, Ala. By virtue of their acidfunctionality, such electrophilically activated PEGs are more readilypurified prior rather than subsequent to reaction with a bifunctionalreactant molecule, NU-Y—S, to allow separation of PEG-diol ordiol-derived impurities. Purification of POLY-E can be accomplished byany of a number of purification methods commonly employed in the art,although chemical-based separation and chromatographic methods arepreferred. One such preferred chromatography method is ion-exchangechromatography or IEC. IEC is useful for the separation of any chargedmolecule, such as a PEG-acid. Typical ion exchange chromatographyconditions can be readily determined by one of skill in the art, such asthe particular column, pH range employed, ionic strength, choice ofbuffer, gradient, and the like.

In some instances, gel permeation chromatography or GPC is utilized todetermine the purity of PEG-containing reactants and derivatives. So, inone instance, GPC may be used to determine the extent of PEG-diol ordiol-derived impurities in a given PEG-starting material or PEGreactant. Once having confirmed the presence and quantity of PEG-diol,e.g., by GPC, the PEG-starting material or derivative, such as aPEG-acid, is then purified by ion exchange chromatography to remove anyPEG-diol or PEG-diol related impurities, such that the resulting PEGcomposition is substantially absent such bifunctional PEG impurities.

Such electrophilically activated PEG reagents are preferablysubstantially pure, i.e., absent PEG-diol or reactive difunctionalPEG-diol derived impurities. Preferably, the starting material,POLY-L_(0,1)-E, will contain less than about 10% of any such impurity,preferably less than about 5% of any such impurity, and more preferablyless than about 2% or no detectable amount of any such impurity.Correspondingly, this means that the resultingthiol-specific-functionalized polymer, POLY-S, preferably comprises atleast 90% or at least 95% or at least 98% or more of the desiredpolymer-based product absent significant amounts of difunctionalizedpolyethylene glycol impurities derived from PEG-diol. Certaindifunctionalized polymer impurities of this type, particularly thecorresponding dithiols or protected dithiols resulting from carryingthrough such impurities when practicing the method of the invention, canbe very difficult to remove. Such reactive impurities, if carriedthrough the reaction scheme to the final activated polymer derivative,can then react with target coupling moieties such as thiol-containinggroups in proteins or other active agents, to provide polymer conjugatesin addition to those intended.

Thiol-Selective Reactant Molecule

In accordance with the method of the invention, POLY-L_(0,1)-E isreacted with a reactant molecule that contains both a nucleophile (—NU)for reaction with the electrophilic group of the activated polymer and athiol-selective group as described above. Generally, a molecularreactant for use in the invention will possess the structure NU-Y—Swhere NU is a nucleophile, Y is a group interposed between NU and S, andS is a thiol-selective group.

“Y”

Y is typically but is not necessarily linear in nature. The overalllength of the Y group will typically range between 1 to about 20 atoms,or from about 2 to 15 atoms, where by length is meant the number ofatoms in a single chain, not counting substituents. For instance, —CH₂—counts as one atom with respect to overall linker length, —CH₂CH₂O—counts as 3 atoms in length. Preferably, Y has a length of about 1 toabout 20 atoms, or from about 2 to about 15 atoms, or from about 1 toabout 6 atoms, and is hydrolytically stable.

Representative Y groups may be any of the following: —CH₂—, —CH₂—CH₂—,—CH₂—CH₂—CH₂—, —CH₂—CH₂—CH₂—CH₂—, —O—CH₂—, —CH₂—O—, —O—CH₂—CH₂—,—CH₂—O—CH₂—, —CH₂—CH₂—O—, —O—CH₂—CH₂—CH₂—, —CH₂—O—CH₂—CH₂—,—CH₂—CH₂—O—CH₂—, —CH₂—CH₂—CH₂—O—, —O—CH₂—CH₂—CH₂—CH₂—,—CH₂—O—CH₂—CH₂—CH₂—, —CH₂—CH₂—CH₂—CH₂—, —CH₂—CH₂—CH₂—O—CH₂—,—CH₂—CH₂—CH₂—CH₂—O—, —C(O)—NH—CH₂—, —C(O)—NH—CH₂—CH₂—,—CH₂—C(O)—NH—CH₂—, —CH₂—CH₂—C(O)—NH—, —C(O)—NH—CH₂—CH₂—CH₂—,—CH₂—C(O)—NH—CH₂—CH₂—, —CH₂—CH₂—C(O)—NH—CH₂—, —CH₂—CH₂—CH₂—C(O)—NH—,—C(O)—NH—CH₂—CH₂—CH₂—CH₂—, —CH₂—C(O)—NH—CH₂—CH₂—CH₂—,—CH₂—CH₂—C(O)—NH—CH₂—CH₂—, —CH₂—CH₂—CH₂—C(O)—NH—CH₂—,—CH₂—CH₂—CH₂—C(O)—NH—CH₂—CH₂—, —CH₂—CH₂—CH₂—CH₂—C(O)—NH—, —C(O)—O—CH₂—,—CH₂—C(O)—O—CH₂—, —CH₂—CH₂—C(O)—O—CH₂—, —C(O)—O—CH₂—CH₂—, —NH—C(O)—CH₂—,—CH₂—NH—C(O)—CH₂—, —CH₂—CH₂—NH—C(O)—CH₂—, —NH—C(O)—CH₂—CH₂—,—CH₂—NH—C(O)—CH₂—CH₂, —CH₂—CH₂—NH—C(O)—CH₂—CH₂, —C(O)—NH—CH₂—,—C(O)—NH—CH₂—CH₂—, —O—C(O)—NH—CH₂—, —O—C(O)—NH—CH₂—CH₂—, —NH—CH₂—,—NH—CH₂—CH₂—, —CH₂—NH—CH₂—, —CH₂—CH₂—NH—CH₂—, —C(O)—CH₂—,—C(O)—CH₂—CH₂—, —CH₂—C(O)—CH₂—, —CH₂—CH₂—C(O)—CH₂—,—CH₂—CH₂—C(O)—CH₂—CH₂—, —CH₂—CH₂—C(O)—,—CH₂—CH₂—CH₂—C(O)—NH—CH₂—CH₂—NH—, —CH₂—CH₂—CH₂—C(O)—NH—CH₂—CH₂—NH—C(O)—,—CH₂—CH₂—CH₂—C(O)—NH—CH₂—CH₂—NH—C(O)—CH₂—, a cycloalkylene group, or asubstituted cycloalkylene group,—CH₂—CH₂—CH₂—C(O)—NH—CH₂—CH₂—NH—C(O)—CH₂—CH₂—, and combinations of twoor more of any of the foregoing.

Preferred Y groups for use in the invention include, for example,alkylene, substituted alkylene, cycloalkylene, substitutedcycloalkylene, aryl, and substituted aryl. Y typically comprises fromabout 2 to about 10 carbon atoms, and optionally may contain additionalnon-interfering atoms. Illustrative Y groups possess the structure:

where R¹ and R² in each occurrence are each independently H or anorganic radical that is selected from the group consisting of alkyl,substituted alkyl, cycloalkyl, substituted cycloalkyl,alkylenecycloalkyl, cycloalkylene, substituted cycloalkylene, andsubstituted alkylenecycloalkyl. Preferably, Y is composed of from two toabout ten carbon atoms. Exemplary Y groups include methylene (—CH₂₋),ethylene (—CH₂CH₂₋), propylene (—CH₂CH₂CH₂₋), butylene (—CH₂CH₂CH₂CH₂—),pentylene (—CH₂CH₂CH₂CH₂CH₂—), 2-methylpropyl, substituted counterparts,and the like. In a particular embodiment of the above structure, R¹ andR² are both H.

Another preferred Y group possesses the structure:—(CH₂)_(1,2,3,4,5)—NH—C(O)—CH₂CH₂—.

Thiol-Selective Group, “S”

Exemplary thiol-selective groups include thiol, protected thiol,disulfide, maleimide, vinylsulfone, iodoacetamide, and orthopyridyldisulfide. “S” as set forth herein represents any thiol-selective group.Particularly, “S” may represent a thiol, thiolate, disulfide or otherprotected thiol group. Protecting groups for the thiol moiety, besidesdisulfide, include trityl, thioethers such as alkyl and benzylthioethers, including monothio, dithio and aminothio acetals,thioesters, thiocarbonates, thiocarbamates, and sulfenyl derivatives.Structures corresponding to these exemplary “S” groups are providedbelow, where a dotted line indicates a point of attachment to the Yportion of the molecule. A-SH indicates an active agent having a thiolgroup.

Thiol-selective group Corresponding A-SH Conjugate

The Nucleophile

The nucleophile portion of the reactant molecule is any nucleophilecommonly known in the art. Preferred nucleophiles include primary amino,secondary amino, hydroxy, imino, thiol, thioester and the like.Secondary amino groups will typically possess as a substituent a loweralkyl group such as methyl, ethyl.

Thus, molecular reactant molecules for use in the invention possess anycombination of NU, Y, and S groups provided herein. Preferred molecularreactants include cystamine, an illustrative symmetrical amino disulfidecompound, and cysteamine, an amino thiol, as well as isN-(2-amino-ethyl)-3-maleimido-propionamide. Reactions carried out withthe exemplary molecular reactant, cystamine, are provided in Examples1-3 and an exemplary reaction scheme with the molecular reactant,N-(2-amino-ethyl)-3-maleimido-propionamide, is provided below.

Reaction Conditions

The reaction between the electrophilic group of the polymer and thenucleophile in the molecular reactant is typically although notnecessarily carried out under mild reaction conditions, depending ofcourse on the particular electrophile and nucleophile that areundergoing reaction. Typically, such reactions are conducted attemperatures at around 100° C. or less, or at 65° C. or less, or at 40°C. or less, or at about 25° C. or less. The reacting step is typicallycarried out in an organic solvent such as acetone, acetonitrile,chlorinated hydrocarbons such as chloroform and dichloromethane,aromatic hydrocarbons such as benzene, toluene or xylene,tetrahydrofuran (THF), dimethylformamide (DMF), or dimethylsulfoxide.

Particular reaction conditions (solvent, molar ratios of reactants,temperature, atmosphere, reaction times) will be readily determined byone skilled in the art, depending upon the choice of particularreactants and the desired products. The course or progress of thereaction can be monitored by any of a number of common analyticaltechniques such as thin layer chromatography or ¹H NMR.

In instances in which the thiol-selective moiety is a protected thiol,an additional deprotection step may be required. Conditions fordeprotection will depend upon the nature of the protecting group, andsuch can readily be determined by one skilled in the art, e.g., asdescribed in Greene, T., and Wuts, Peter G. M., “PROTECTIVE GROUPS INORGANIC SYNTHESIS, Chapter 6, 3^(rd) Edition, John Wiley and Sons, Inc.,New York, 1999 (p. 454-493).

When a symmetrical disulfide reagent such as cystamine is employed asthe molecular reactant, the resulting polymer product is a symmetricalpolymer having a central disulfide bond. Representative symmetricalwater-soluble polymers having a central disulfide bond are provided asstructures (10) and (12), although many alternative structures havingthese basic features may readily be envisioned, based upon thedescriptions of the reaction and POLY, L₁, E, NU, Y, and S groupsprovided herein. The symmetrical polymer disulfide can then be convertedto the corresponding thiol-terminated polymer, e.g., (11) and (13), andthe like, by reduction with a suitable reducing agent such asdithiothreitol, Sn/HCl, Na/xylene, ammonia, lithium aluminum hydride,sodium borohydride or any other reducing agent known in the art.

Thiol-Selective Polymers

In another aspect, the invention also provides thiol-selective polymershaving the features and components described above. Generally, athiol-selective polymer of the possesses the following structure:POLY-L_(0,1)-X-Y—S, where the variables L, X, Y and S have beenpreviously described. All of the above exemplary POLYs, linkers, Ygroups, and S groups are encompassed by the generalized structure for athiol-selective polymer of the invention above. Preferably, X, afunctional group resulting from the reaction of the electrophile of thepolymer reagent and the nucleophile on the molecular reactant, is eitheran amide (—C(O)—NH—, or a urethane, —O—C(O)—NH—. In some instances, thefunctional group X is designated herein as “-G₁-C(O)-G₂-”, where G₁ andG₂ are each independently a heteroatom such as O, NH, or S. In oneembodiment, G₁ is absent, and Y corresponds to C(O)-G. Preferably, G₂ is—NH. Symmetrical polymer disulfides of the invention possess thegeneralized structure: (POLY-L_(0,1)-X—Y—S—)₂, II, which encompasses allof the herein described POLYs, linkers, and Y groups.

Storage of Polymer Reagents

Preferably, the thiol-selective polymers of the invention are storedunder an inert atmosphere, such as under argon or under nitrogen. It isalso preferable to minimize exposure of the polymers of the invention tomoisture. Thus, preferred storage conditions are under dry argon oranother dry inert gas at temperatures below about −15° C. Storage underlow temperature conditions is preferred, since rates of undesirable sidereactions are slowed at lower temperatures. In instances where thepolymer segment of the polymer product is PEG, the PEG portion can reactslowly with oxygen to form peroxides along the PEG portion of themolecule. Formation of peroxides can ultimately lead to chain cleavage,thus increasing the polydispersity of the PEG reagents provided herein.In view of the above, it is additionally preferred to store the polymersof the invention in the dark.

Thiol-Activated Polymer Conjugates

The present invention also encompasses conjugates formed by reaction ofany of the herein described thiol-selective polymers. In particular, thethiol-selective polymers of the invention are useful for conjugation toactive agents or surfaces bearing at least one thiol or amino groupavailable for reaction. Conjugates will possess the structurecorresponding to functional groups formed by reacting any of the hereindescribed thiol-selective groups, e.g., thiol, maleimide, vinylsulfone,orthopyridyldisulfide, with an accessible thiol contained in an activeagent.

For instance, a conjugate of the invention may possess the followingstructure: POLY-L_(0,1)-X-Y—S—S-active agent, IV, where S—S— is adisulfide bond.

Alternatively, a conjugate of the invention may possess the followingstructure:

where “—S-active agent” represents an active agent, preferably abiologically active agent, comprising a thiol (—SH) group, and the othervariables are as described previously. In instances where the activeagent is a biologically active agent or small molecule containing onlyone reactive thiol group, the resulting composition may advantageouslycontain only a single polymer conjugate species, due to the relativelylow number of sulfhydryl groups typically contained within a protein andaccessible for conjugation. In some instances, a protein or smallmolecule or other active agent is engineered to possess a thiol group ina known position, and will similarly result in a composition comprisingonly a single polymer conjugate species. This approach is generallyreferred to as site-specific modification.

Alternatively, a conjugate of the invention may possess the followingstructure:

In the above structure, “—NH-active agent” represents an active agent orsurface comprising an amino group, preferably a biologically activeagent, and the other variables are as previously described. Undercertain reaction conditions, maleimide groups can react with aminogroups, such as those present in an active agent such as a protein.

A cysteine residue for coupling to an activated polymer of the inventionmay be naturally occurring (i.e., occurs in the protein in its nativeform) or may be inserted into the native sequence in place of anaturally-occurring amino acid using standard genetic engineeringtechniques. Since thiol groups are less numerous in proteins than areother typical polymer attachment sites such as amino groups, covalentattachment of a polymer derivative can result in more selectivepegylation of the target protein. That is to say, the polymerderivatives of the invention can allow greater control over theresulting polymer conjugate-both in the number of polymer derivativesattached to the parent protein (mono versus di-versus tri-substitutedconjugates, etc.) and the position of polymer attachment.

The generalized features of the conjugates of the invention have beendescribed in detailed fashion above. Active agents that are covalentlyattached to a thiol-selective polymer encompass any of a number of typesof molecules, entities, surfaces, and the like, as will become apparentfrom the following.

Target Molecules and Surfaces

The thiol-selective polymers of the invention may be attached, eithercovalently or non-covalently, to a number of entities including films,chemical separation and purification surfaces, solid supports,metal/metal oxide surfaces such as gold, titanium, tantalum, niobium,aluminum, steel, and their oxides, silicon oxide, macromolecules, andsmall molecules. Additionally, the polymers and methods of the inventionmay also be used in biochemical sensors, bioelectronic switches, andgates. The polymers and methods of the invention may also be employed inpreparing carriers for peptide synthesis, for the preparation ofpolymer-coated surfaces and polymer grafts, to prepare polymer-ligandconjugates for affinity partitioning, to prepare cross-linked ornon-cross-linked hydrogels, and to prepare polymer-cofactor adducts forbioreactors.

A biologically active agent for use in providing a conjugate of theinvention may be any one or more of the following. Suitable agents maybe selected from, for example, hypnotics and sedatives, psychicenergizers, tranquilizers, respiratory drugs, anticonvulsants, musclerelaxants, antiparkinson agents (dopamine antagnonists), analgesics,anti-inflammatories, antianxiety drugs (anxiolytics), appetitesuppressants, antimigraine agents, muscle contractants, anti-infectives(antibiotics, antivirals, antifungals, vaccines) antiarthritics,antimalarials, antiemetics, anepileptics, bronchodilators, cytokines,growth factors, anti-cancer agents, antithrombotic agents,antihypertensives, cardiovascular drugs, antiarrhythmics, antioxicants,anti-asthma agents, hormonal agents including contraceptives,sympathomimetics, diuretics, lipid regulating agents, antiandrogenicagents, antiparasitics, anticoagulants, neoplastics, antineoplastics,hypoglycemics, nutritional agents and supplements, growth supplements,antienteritis agents, vaccines, antibodies, diagnostic agents, andcontrasting agents.

More particularly, the active agent may fall into one of a number ofstructural classes, including but not limited to small molecules(preferably insoluble small molecules), peptides, polypeptides,proteins, antibodies, polysaccharides, steroids, nucleotides,oligonucleotides, polynucleotides, fats, electrolytes, and the like.Preferably, an active agent for coupling to a polymer of the inventionpossesses a native sulfydryl group or less preferably a native aminogroup, or alternatively, is modified to contain at least one reactivesulfhydryl group or amino group suitable for coupling.

Specific examples of active agents include but are not limited toaspariginase, amdoxovir (DAPD), antide, becaplermin, calcitonins,cyanovirin, denileukin diftitox, erythropoietin (EPO), EPO agonists(e.g., peptides from about 10-40 amino acids in length and comprising aparticular core sequence as described in WO 96/40749), dornase alpha,erythropoiesis stimulating protein (NESP), coagulation factors such asFactor V, Factor VII, Factor VIIa, Factor VIII, Factor IX, Factor X,Factor XII, Factor XIII, von Willebrand factor; ceredase, cerezyme,alpha-glucosidase, collagen, cyclosporin, alpha defensins, betadefensins, exedin-4, granulocyte colony stimulating factor (GCSF),thrombopoietin (TPO), alpha-1 proteinase inhibitor, elcatonin,granulocyte macrophage colony stimulating factor (GMCSF), fibrinogen,filgrastim, growth hormones human growth hormone (hGH), growth hormonereleasing hormone (GHRH), GRO-beta, GRO-beta antibody, bone morphogenicproteins such as bone morphogenic protein-2, bone morphogenic protein-6,OP-1; acidic fibroblast growth factor, basic fibroblast growth factor,CD-40 ligand, heparin, human serum albumin, low molecular weight heparin(LMWH), interferons such as interferon alpha, interferon beta,interferon gamma, interferon omega, interferon tau, consensusinterferon; interleukins and interleukin receptors such as interleukin-1receptor, interleukin-2, interleukin-2 fusion proteins, interleukin-1receptor antagonist, interleukin-3, interleukin-4, interleukin-4receptor, interleukin-6, interleukin-8, interleukin-12, interleukin-13receptor, interleukin-17 receptor; lactoferrin and lactoferrinfragments, luteinizing hormone releasing hormone (LHRH), insulin,pro-insulin, insulin analogues (e.g., mono-acylated insulin as describedin U.S. Pat. No. 5,922,675), amylin, C-peptide, somatostatin,somatostatin analogs including octreotide, vasopressin, folliclestimulating hormone (FSH), influenza vaccine, insulin-like growth factor(IGF), insulintropin, macrophage colony stimulating factor (M-CSF),plasminogen activators such as alteplase, urokinase, reteplase,streptokinase, pamiteplase, lanoteplase, and teneteplase; nerve growthfactor (NGF), osteoprotegerin, platelet-derived growth factor, tissuegrowth factors, transforming growth factor-1, vascular endothelialgrowth factor, leukemia inhibiting factor, keratinocyte growth factor(KGF), glial growth factor (GGF), T Cell receptors, CDmolecules/antigens, tumor necrosis factor (TNF), monocytechemoattractant protein-1, endothelial growth factors, parathyroidhormone (PTH), glucagon-like peptide, somatotropin, thymosin alpha 1,thymosin alpha 1 Hb/IIIa inhibitor, thymosin beta 10, thymosin beta 9,thymosin beta 4, alpha-1 antitrypsin, phosphodiesterase (PDE) compounds,VLA-4 (very late antigen-4), VLA-4 inhibitors, bisphosphonates,respiratory syncytial virus antibody, cystic fibrosis transmembraneregulator (CFTR) gene, deoxyreibonuclease (Dnase),bactericidal/permeability increasing protein (BPI), and anti-CMVantibody. Exemplary monoclonal antibodies include etanercept (a dimericfusion protein consisting of the extracellular ligand-binding portion ofthe human 75 kD TNF receptor linked to the Fc portion of IgG1),abciximab, afeliomomab, basiliximab, daclizumab, infliximab, ibritumomabtiuexetan, mitumomab, muromonab-CD3, iodine 131 tositumomab conjugate,olizumab, rituximab, and trastuzumab (herceptin).

Additional agents suitable for covalent attachment to a polymer includebut are not limited to amifostine, amiodarone, aminocaproic acid,aminohippurate sodium, aminoglutethimide, aminolevulinic acid,aminosalicylic acid, amsacrine, anagrelide, anastrozole, asparaginase,anthracyclines, bexarotene, bicalutamide, bleomycin, buserelin,busulfan, cabergoline, capecitabine, carboplatin, carmustine,chlorambucin, cilastatin sodium, cisplatin, cladribine, clodronate,cyclophosphamide, cyproterone, cytarabine, camptothecins, 13-cisretinoic acid, all trans retinoic acid; dacarbazine, dactinomycin,daunorubicin, deferoxamine, dexamethasone, diclofenac,diethylstilbestrol, docetaxel, doxorubicin, epirubicin, estramustine,etoposide, exemestane, fexofenadine, fludarabine, fludrocortisone,fluorouracil, fluoxymesterone, flutamide, gemcitabine, epinephrine,L-Dopa, hydroxyurea, idarubicin, ifosfamide, imatinib, irinotecan,itraconazole, goserelin, letrozole, leucovorin, levamisole, lisinopril,lovothyroxine sodium, lomustine, mechlorethamine, medroxyprogesterone,megestrol, melphalan, mercaptopurine, metaraminol bitartrate,methotrexate, metoclopramide, mexiletine, mitomycin, mitotane,mitoxantrone, naloxone, nicotine, nilutamide, octreotide, oxaliplatin,pamidronate, pentostatin, pilcamycin, porfimer, prednisone,procarbazine, prochlorperazine, ondansetron, raltitrexed, sirolimus,streptozocin, tacrolimus, tamoxifen, temozolomide, teniposide,testosterone, tetrahydrocannabinol, thalidomide, thioguanine, thiotepa,topotecan, tretinoin, valrubicin, vinblastine, vincristine, vindesine,vinorelbine, dolasetron, granisetron; formoterol, fluticasone,leuprolide, midazolam, alprazolam, amphotericin B, podophylotoxins,nucleoside antivirals, aroyl hydrazones, sumatriptan; macrolides such aserythromycin, oleandomycin, troleandomycin, roxithromycin,clarithromycin, davercin, azithromycin, flurithromycin, dirithromycin,josamycin, spiromycin, midecamycin, leucomycin, miocamycin, rokitamycin,andazithromycin, and swinolide A; fluoroquinolones such asciprofloxacin, ofloxacin, levofloxacin, trovafloxacin, alatrofloxacin,moxifloxicin, norfloxacin, enoxacin, grepafloxacin, gatifloxacin,lomefloxacin, sparfloxacin, temafloxacin, pefloxacin, amifloxacin,fleroxacin, tosufloxacin, prulifloxacin, ofloxacin, pazufloxacin,clinafloxacin, and sitafloxacin; aminoglycosides such as gentamicin,netilmicin, paramecin, tobramycin, amikacin, kanamycin, neomycin, andstreptomycin, vancomycin, teicoplanin, rampolanin, mideplanin, colistin,daptomycin, gramicidin, colistimethate; polymixins such as polymixin B,capreomycin, bacitracin, penems; penicillins includingpenicllinase-sensitive agents like penicillin G, penicillin V;penicllinase-resistant agents like methicillin, oxacillin, cloxacillin,dicloxacillin, floxacillin, nafcillin; gram negative microorganismactive agents like ampicillin, amoxicillin, and hetacillin, cillin, andgalampicillin; antipseudomonal penicillins like carbenicillin,ticarcillin, azlocillin, mezlocillin, and piperacillin; cephalosporinslike cefpodoxime, cefprozil, ceftbuten, ceftizoxime, ceftriaxone,cephalothin, cephapirin, cephalexin, cephradrine, cefoxitin,cefamandole, cefazolin, cephaloridine, cefaclor, cefadroxil,cephaloglycin, cefuroxime, ceforanide, cefotaxime, cefatrizine,cephacetrile, cefepime, cefixime, cefonicid, cefoperazone, cefotetan,cefmetazole, ceftazidime, loracarbef, and moxalactam, monobactams likeaztreonam; and carbapenems such as imipenem, meropenem, pentamidineisethiouate, albuterol sulfate, lidocaine, metaproterenol sulfate,beclomethasone diprepionate, triamcinolone acetamide, budesonideacetonide, fluticasone, ipratropium bromide, flunisolide, cromolynsodium, and ergotamine tartrate; taxanes such as paclitaxel; SN-38, andtyrphostines.

Preferred peptides or proteins for coupling to a thiol-selective polymerof the invention include EPO, IFN-α, IFN-β, IFN-γ, consensus IFN, FactorVII, Factor VIII, Factor IX, IL-2, remicade (infliximab), Rituxan(rituximab), Enbrel (etanercept), Synagis (palivizumab), Reopro(abciximab), Herceptin (trastuzimab), tPA, Cerizyme (imiglucerase),Hepatitus-B vaccine, rDNAse, alpha-1 proteinase inhibitor, GCSF, GMCSF,hGH, insulin, FSH, and PTH.

The above exemplary biologically active agents are meant to encompass,where applicable, analogues, agonists, antagonists, inhibitors, isomers,and pharmaceutically acceptable salt forms thereof. In reference topeptides and proteins, the invention is intended to encompass synthetic,recombinant, native, glycosylated, and non-glycosylated forms, as wellas biologically active fragments thereof. The above biologically activeproteins are additionally meant to encompass variants having one or moreamino acids substituted (e.g., cysteine), deleted, or the like, as longas the resulting variant protein possesses at least a certain degree ofactivity of the parent (native) protein.

The conjugates or methods described herein can also be extended tohydrogel formulations.

Methods of Conjugation

Suitable conjugation conditions are those conditions of time,temperature, pH, reagent concentration, solvent, and the like sufficientto effect conjugation between a polymeric reagent and an active agent.As is known in the art, the specific conditions depend upon, among otherthings, the active agent, the type of conjugation desired, the presenceof other materials in the reaction mixture and so forth. Sufficientconditions for effecting conjugation in any particular case can bedetermined by one of ordinary skill in the art upon a reading of thedisclosure herein, reference to the relevant literature, and/or throughroutine experimentation.

Exemplary conjugation conditions include carrying out the conjugationreaction at a pH of from about 6 to about 10, and at, for example, a pHof about 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, or 10. The reaction isallowed to proceed from about 5 minutes to about 72 hours, preferablyfrom about 30 minutes to about 48 hours, and more preferably from about4 hours to about 24 hours or less. Temperatures for conjugationreactions are typically, although not necessarily, in the range of fromabout 0° C. to about 40° C.; conjugation is often carried out at roomtemperature or less. Conjugation reactions are often carried out in abuffer such as a phosphate or acetate buffer or similar system.

With respect to reagent concentration, an excess of the polymericreagent is typically combined with the active agent. In some cases,however, it is preferred to have stoichiometic amounts of the number ofreactive groups on the polymeric reagent to the amount of active agent.Exemplary ratios of polymeric reagent to active agent include molarratios of about 1:1 (polymeric reagent:active agent), 1.5:1, 2:1, 3:1,4:1, 5:1, 6:1, 8:1, or 10:1. The conjugation reaction is allowed toproceed until substantially no further conjugation occurs, which cangenerally be determined by monitoring the progress of the reaction overtime.

Progress of the reaction can be monitored by withdrawing aliquots fromthe reaction mixture at various time points and analyzing the reactionmixture by SDS-PAGE or MALDI-TOF mass spectrometry or any other suitableanalytical method. Once a plateau is reached with respect to the amountof conjugate formed or the amount of unconjugated polymer remaining, thereaction is assumed to be complete. Typically, the conjugation reactiontakes anywhere from minutes to several hours (e.g., from 5 minutes to 24hours or more). The resulting product mixture is preferably, but notnecessarily purified, to separate out excess reagents, unconjugatedreactants (e.g., active agent) undesired multi-conjugated species, andfree or unreacted polymer. The resulting conjugates can then be furthercharacterized using analytical methods such as MALDI, capillaryelectrophoresis, gel electrophoresis, and/or chromatography.

More preferably, a thiol-selective polymer of the invention is typicallyconjugated to a sulfhydryl-containing active agent at pHs ranging fromabout 6-9 (e.g., at 6, 6.5, 7, 7.5, 8, 8.5, or 9), more preferably atpHs from about 7-9, and even more preferably at pHs from about 7 to 8.Generally, a slight molar excess of polymer reagent is employed, forexample, a 1.5 to 15-fold molar excess, preferably a 2-fold to 10 foldmolar excess. Reaction times generally range from about 15 minutes toseveral hours, e.g., 8 or more hours, at room temperature. Forsterically hindered sulfhydryl groups, required reaction times may besignificantly longer. Since the polymers of the invention arethiol-selective, thiol-selective conjugation is preferably conducted atpHs around 7.

Separation

Optionally, conjugates produced by reacting a thiol-selective polymer ofthe invention with a biologically active agent are purified toobtain/isolate different species, e.g., PEG— species, or to removeundesirable reaction side-products.

If desired, PEG conjugates having different molecular weights can beisolated using gel filtration chromatography. While this approach can beused to separate PEG conjugates having different molecular weights, thisapproach is generally ineffective for separating positional isomershaving different pegylation sites within a protein. For example, gelfiltration chromatography can be used to separate from each othermixtures of PEG 1-mers, 2-mers, 3-mers, etc., although each of therecovered PEG-mer compositions may contain PEGs attached to differentreactive groups within the protein.

Gel filtration columns suitable for carrying out this type of separationinclude Superdex™ and Sephadex™ columns available from AmershamBiosciences. Selection of a particular column will depend upon thedesired fractionation range desired. Elution is generally carried outusing a non-amine based buffer, such as phosphate, acetate, or the like.The collected fractions may be analysed by a number of differentmethods, for example, (i) OD at 280 nm for protein content, (ii) BSAprotein analysis, (iii) iodine testing for PEG content (Sims G. E. C.,et al., Anal. Biochem, 107, 60-63, 1980), or alternatively, (iv) byrunning an SDS PAGE gel, followed by staining with barium iodide.

Separation of positional isomers can be carried out by reverse phasechromatography using, for example, an RP-HPLC C18 column (AmershamBiosciences or Vydac) or by ion exchange chromatography using an ionexchange column, e.g., a Sepharose™ ion exchange column available fromAmersham Biosciences. Either approach can be used to separatePEG-biomolecule isomers having the same molecular weight (positionalisomers).

Depending upon the intended use for the resulting PEG-conjugates,following conjugation, and optionally additional separation steps, theconjugate mixture may be concentrated, sterile filtered, and stored atlow temperatures from about −20° C. to about −80° C. Alternatively, theconjugate may be lyophilized, either with or without residual buffer andstored as a lyophilized powder. In some instances, it is preferable toexchange a buffer used for conjugation, such as sodium acetate, for avolatile buffer such as ammonium carbonate or ammonium acetate, that canbe readily removed during lyophilization, so that the lyophilizedprotein conjugate powder formulation is absent residual buffer.Alternatively, a buffer exchange step may be used using a formulationbuffer, so that the lyophilized conjugate is in a form suitable forreconstitution into a formulation buffer and ultimately foradministration to a mammal.

Pharmaceutical Compositions

The present invention also includes pharmaceutical preparationscomprising a conjugate as provided herein in combination with apharmaceutical excipient. Generally, the conjugate itself will be in asolid form (e.g., a precipitate) or in solution, which can be combinedwith a suitable pharmaceutical excipient that can be in either solid orliquid form.

Exemplary excipients include, without limitation, those selected fromthe group consisting of carbohydrates, inorganic salts, antimicrobialagents, antioxidants, surfactants, buffers, acids, bases, andcombinations thereof.

A carbohydrate such as a sugar, a derivatized sugar such as an alditol,aldonic acid, an esterified sugar, and/or a sugar polymer may be presentas an excipient. Specific carbohydrate excipients include, for example:monosaccharides, such as fructose, maltose, galactose, glucose,D-mannose, sorbose, and the like; disaccharides, such as lactose,sucrose, trehalose, cellobiose, and the like; polysaccharides, such asraffinose, melezitose, maltodextrins, dextrans, starches, and the like;and alditols, such as mannitol, xylitol, maltitol, lactitol, xylitol,sorbitol (glucitol), pyranosyl sorbitol, myoinositol, and the like.

The excipient can also include an inorganic salt or buffer such ascitric acid, sodium chloride, potassium chloride, sodium sulfate,potassium nitrate, sodium phosphate monobasic, sodium phosphate dibasic,and combinations thereof.

The preparation may also include an antimicrobial agent for preventingor deterring microbial growth. Nonlimiting examples of antimicrobialagents suitable for the present invention include benzalkonium chloride,benzethonium chloride, benzyl alcohol, cetylpyridinium chloride,chlorobutanol, phenol, phenylethyl alcohol, phenylmercuric nitrate,thimersol, and combinations thereof.

An antioxidant can be present in the preparation as well. Antioxidantsare used to prevent oxidation, thereby preventing the deterioration ofthe conjugate or other components of the preparation. Suitableantioxidants for use in the present invention include, for example,ascorbyl palmitate, butylated hydroxyanisole, butylated hydroxytoluene,hypophosphorous acid, monothioglycerol, propyl gallate, sodiumbisulfite, sodium formaldehyde sulfoxylate, sodium metabisulfite, andcombinations thereof.

A surfactant may be present as an excipient. Exemplary surfactantsinclude: polysorbates, such as “Tween 20” and “Tween 80,” and pluronicssuch as F68 and F88 (both of which are available from BASF, Mount Olive,N.J.); sorbitan esters; lipids, such as phospholipids such as lecithinand other phosphatidylcholines, phosphatidylethanolamines (althoughpreferably not in liposomal form), fatty acids and fatty esters;steroids, such as cholesterol; and chelating agents, such as EDTA, zincand other such suitable cations.

Acids or bases may be present as an excipient in the preparation.Nonlimiting examples of acids that can be used include those acidsselected from the group consisting of hydrochloric acid, acetic acid,phosphoric acid, citric acid, malic acid, lactic acid, formic acid,trichloroacetic acid, nitric acid, perchloric acid, phosphoric acid,sulfuric acid, fumaric acid, and combinations thereof. Examples ofsuitable bases include, without limitation, bases selected from thegroup consisting of sodium hydroxide, sodium acetate, ammoniumhydroxide, potassium hydroxide, ammonium acetate, potassium acetate,sodium phosphate, potassium phosphate, sodium citrate, sodium formate,sodium sulfate, potassium sulfate, potassium fumerate, and combinationsthereof.

The pharmaceutical preparations encompass all types of formulations andin particular those that are suited for injection, e.g., powders thatcan be reconstituted as well as suspensions and solutions. The amount ofthe conjugate (i.e., the conjugate formed between the active agent andthe polymer described herein) in the composition will vary depending ona number of factors, but will optimally be a therapeutically effectivedose when the composition is stored in a unit dose container (e.g., avial). In addition, the pharmaceutical preparation can be housed in asyringe. A therapeutically effective dose can be determinedexperimentally by repeated administration of increasing amounts of theconjugate in order to determine which amount produces a clinicallydesired endpoint.

The amount of any individual excipient in the composition will varydepending on the activity of the excipient and particular needs of thecomposition. Typically, the optimal amount of any individual excipientis determined through routine experimentation, i.e., by preparingcompositions containing varying amounts of the excipient (ranging fromlow to high), examining the stability and other parameters, and thendetermining the range at which optimal performance is attained with nosignificant adverse effects.

Generally, however, the excipient will be present in the composition inan amount of about 1% to about 99% by weight, preferably from about5%-98% by weight, more preferably from about 15-95% by weight of theexcipient, with concentrations less than 30% by weight most preferred.

These foregoing pharmaceutical excipients along with other excipientsare described in “Remington: The Science & Practice of Pharmacy”,19^(th) ed., Williams & Williams, (1995), the “Physician's DeskReference”, 52^(nd) ed., Medical Economics, Montvale, N.J. (1998), andKibbe, A. H., Handbook of Pharmaceutical Excipients, 3^(rd) Edition,American Pharmaceutical Association, Washington, D.C., 2000.

The pharmaceutical preparations of the present invention are typically,although not necessarily, administered via injection and are thereforegenerally liquid solutions or suspensions immediately prior toadministration. The pharmaceutical preparation can also take other formssuch as syrups, creams, ointments, tablets, powders, and the like. Othermodes of administration are also included, such as pulmonary, rectal,transdermal, transmucosal, oral, intrathecal, subcutaneous,intra-arterial, and so forth.

As previously described, the conjugates can be administered injectedparenterally by intravenous injection, or less preferably byintramuscular or by subcutaneous injection. Suitable formulation typesfor parenteral administration include ready-for-injection solutions, drypowders for combination with a solvent prior to use, suspensions readyfor injection, dry insoluble compositions for combination with a vehicleprior to use, and emulsions and liquid concentrates for dilution priorto administration, among others.

Methods of Administering

The invention also provides a method for administering a conjugate asprovided herein to a patient suffering from a condition that isresponsive to treatment with conjugate. The method comprisesadministering, generally via injection, a therapeutically effectiveamount of the conjugate (preferably provided as part of a pharmaceuticalpreparation). The method of administering may be used to treat anycondition that can be remedied or prevented by administration of theparticular conjugate. Those of ordinary skill in the art appreciatewhich conditions a specific conjugate can effectively treat. The actualdose to be administered will vary depend upon the age, weight, andgeneral condition of the subject as well as the severity of thecondition being treated, the judgment of the health care professional,and conjugate being administered. Therapeutically effective amounts areknown to those skilled in the art and/or are described in the pertinentreference texts and literature. Generally, a therapeutically effectiveamount will range from about 0.001 mg to 100 mg, preferably in dosesfrom 0.01 mg/day to 75 mg/day, and more preferably in doses from 0.10mg/day to 50 mg/day.

The unit dosage of any given conjugate (again, preferably provided aspart of a pharmaceutical preparation) can be administered in a varietyof dosing schedules depending on the judgment of the clinician, needs ofthe patient, and so forth. The specific dosing schedule will be known bythose of ordinary skill in the art or can be determined experimentallyusing routine methods. Exemplary dosing schedules include, withoutlimitation, administration five times a day, four times a day, threetimes a day, twice daily, once daily, three times weekly, twice weekly,once weekly, twice monthly, once monthly, and any combination thereof.Once the clinical endpoint has been achieved, dosing of the compositionis halted.

One advantage of administering the conjugates of the present inventionis that individual water-soluble polymer portions can be cleaved off.Such a result is advantageous when clearance from the body ispotentially a problem because of the polymer size. Optimally, cleavageof each water-soluble polymer portion is facilitated through the use ofphysiologically cleavable and/or enzymatically degradable linkages suchas urethane, amide, carbonate or ester-containing linkages. In this way,clearance of the conjugate (via cleavage of individual water-solublepolymer portions) can be modulated by selecting the polymer molecularsize and the type functional group that would provide the desiredclearance properties. One of ordinary skill in the art can determine theproper molecular size of the polymer as well as the cleavable functionalgroup. For example, one of ordinary skill in the art, using routineexperimentation, can determine a proper molecular size and cleavablefunctional group by first preparing a variety of polymer derivativeswith different polymer weights and cleavable functional groups, and thenobtaining the clearance profile (e.g., through periodic blood or urinesampling) by administering the polymer derivative to a patient andtaking periodic blood and/or urine sampling. Once a series of clearanceprofiles have been obtained for each tested conjugate, a suitableconjugate can be identified.

All articles, books, patents, patent publications and other publicationsreferenced herein are incorporated by reference in their entireties.

The following examples illustrate, but in no way are intended to limitthe scope of the present invention.

EXAMPLES Materials and Methods

¹H NMR data was obtained using a 400 MHz spectrometer manufactured byBruker.

PEG reagents referred to in the appended examples are available fromNektar Therapeutics, Huntsville, Ala.

1. Preparation of mPEG-5K Propionic Acid, N-Hydroxysuccinimide (NHS)Ester

The PEG reagent, mPEG-5K propionic acid, N-hydroxysuccinimide (NHS)ester, was synthesized as follows.

A. M-PEG(5,000)-Nitrile (1)

M-PEG-OH (methoxy-PEG, MW=5,000 daltons, 50 g, containing 4 wt % ofhigher molecular weight PEG-diol, as determined by Gel PermeationChromatography(GPC), was dissolved in distilled water (50.0 ml) to whichwas added potassium hydroxide (1.0 g). The solution was cooled to 0-5°C. in an ice bath. Acrylonitrile (6.8 g) was added slowly, and thesolution was stirred for 2.5 hours at 0-5° C. The pH of the solution wasadjusted to 7 by addition of sodium dihydrogen phosphate. The productwas extracted three times with dichloromethane (250, 100 and 50 ml). Thecombined organic layers were dried over magnesium sulfate, filtered,concentrated and the product was precipitated by addition to ethyl etherat 0-5° C. The precipitate was removed by filtration and dried undervacuum.

Yield 47.0 g. NMR (d₆-DMSO): 2.74 ppm (t, 2H, —CH₂—CN); 3.21 ppm, (s,3H, —OCH₃), 3.51 ppm (s, PEG backbone).

B. M-PEG(5,000)-Amide (2)

A mixture of M-PEG(5,000)-nitrile, (1), (47.0 g) and concentratedhydrochloric acid (235 g) was stirred at room temperature for 48 hours.The solution was diluted with two liters of water and extracted withdichloromethane (300, 200, and 100 ml). The combined organic extractswere washed twice with water, dried over sodium sulfate, filtered, andconcentrated to dryness by rotary evaporation.

Yield 43.0 g. NMR (d₆-DMSO): 2.26 ppm (t, 2H, —CH₂—CONH₂); 2.43 ppm (t,2H, —CH₂ —COOH); 3.21 ppm (s, 3H, —OCH₃), 3.51 ppm (s, PEG backbone).

C. M-PEG(5,000)-Propionic Acid, (Alpha-Methoxy, Omega-Propionic Acid ofPEG) (3)

M-PEG(5,000)-amide (2) (32.0 g) was dissolved in 2300 ml of distilledwater to which was added 200 g of potassium hydroxide, and the solutionwas stirred for 22 hours at room temperature. Sodium chloride (300 g)was added, and the solution was extracted three times each with 300 mldichloromethane. The combined organic extracts were washed with 5%oxalic acid, water (twice), and dried over sodium sulfate. The solutionwas concentrated and the product precipitated by addition to ethylether. The product M-PEG(5,000)-propionic acid (3) was collected byfiltration and dried over vacuum.

Yield 28.0 g. NMR (d₆-DMSO): 2.43 ppm (t, 2H, —CH₂—COOH); 3.21 ppm (s,3H, —OCH₃), 3.51 ppm (s, PEG backbone).

Removal of Difunctional Impurities:

M-PEG(5,000)-propionic acid (3) containing 4 wt % ofPEG(10,000)-dipropionic acid (22 g) (derived from reaction of PEG diolimpurity contained in the starting material) was dissolved in 2200 mldeionized water and the resulting solution was applied to a DEAESephadex A-25 chromatographic column in the tetraborate form. A stepwiseionic gradient of sodium chloride (from 2 to 14 mM at increments) wasapplied, and fraction collection (approx. 60 ml each) begun. Fractions4-25 contained pure M-PEG(5,000)-propionic acid. The subsequent twofractions did not contain PEG, while fractions 28-36 contained the purePEG(10,000)-dipropionic acid. The fractions containing pureM-PEG(5,000)-propionic acid were combined and concentrated (to approx.100 ml). Sodium chloride (10 g) was added, the pH was adjusted to 3 andthe product was extracted with dichloromethane. The organic layer wasdried over MgSO₄, and the solvent was distilled off under reducedpressure to give 18.4 g of product.

HPLC analysis showed that the product was 100% pureM-PEG(5,000)-propionic acid (absent any other impurites).

D. M-PEG(5,000)-Propionic Acid, NHS Ester, (Alpha-Methoxy,Omega-Propionic Acid Succinimidyl Ester of PEG (“Methoxy-PEG-SPA”)), (4)

M-PEG(5,000)-propionic acid (14.4 g), (3), was dissolved indichloromethane (60 ml) to form a solution to which was addedN-hydroxysuccinimide (0.36 g). The solution was cooled to 0° C., and asolution of dicyclohexylcarbodiimide (0.72 g) in 10 ml dichloromethanewas added dropwise. The solution was stirred overnight at roomtemperature under an argon atmosphere. The reaction mixture wasfiltered, concentrated, and the product was precipitated by addition toethyl ether.

Yield of final product (4): 14.0 g. NMR (d₆-DMSO): 2.81 ppm (s, 4H,NHS); 2.92 ppm (t, 2H, —CH₂—COO—); 3.21 ppm, (s, 3H, —OCH₃), 3.51 ppm(s, PEG backbone).

2. Preparation of mPEG-20K Butanoic Acid, N-Hydroxysuccinimide (NHS)Ester

The PEG reagent, mPEG-20K butanoic acid, N-hydroxysuccinimide (NHS)ester, was synthesized as follows.

A. M-PEG(20K)-Methanesulfonate (5)

M-PEG-OH (MW=20,000 daltons, 60 g, containing 6 wt % of higher molecularweight PEG-diol (determined by Gel Permeation Chromatography(GPC))) wasdissolved in 300 ml of toluene and azeotropically distilled for 1 hourunder argon atmosphere. Next the solution was cooled to roomtemperature. To the solution was added 24 ml of anhydrousdichloromethane and 0.62 ml of triethylamine (0.0044 moles). 0.28 ml ofmethanesulfonyl chloride (0.0036 moles) was added dropwise. The solutionwas stirred at room temperature under nitrogen atmosphere overnight.Sodium carbonate (30 g) was then added, and the mixture was stirred for1 h. The solution was filtered and solvents were distilled off underreduced pressure. Yield 27.5 g

¹H NNR (d₆-DMSO): 3.17 ppm (s, 3H, CH₃— methanesulfonate), 3.24 ppm (s,3H, —OCH₃), 3.51 ppm (s, PEG backbone), 4.30 ppm (m, —CH₂—methanesulfonate).

B. M-PEG(20,000)-Butanoic Acid (8)

Ethyl malonate (3.4 ml, 0.022 equivalents) dissolved in 200 ml ofdioxane was added drop by drop to sodium hydride (0.536 g, 0.022equivalents) and toluene (100 ml) in a round bottomed flask undernitrogen. M-PEG(20K)-methanesulfonate (5) (40 g, 0.0020 moles) dissolvedin 100 ml of toluene was added to the above mixture. The resultingmixture was refluxed overnight. The reaction mixture was thenconcentrated to half its original volume, extracted with 50 ml of 10%aqueous NaCl solution, extracted with 50 ml of 1% aqueous hydrochloricacid, and the aqueous extracts combined. The collected aqueous layerswere extracted with dichloromethane (150 ml×3), and the organic layerwas dried over magnesium sulfate for 3 hours, filtered, and evaporatedto dryness.

Yield: 36 g of M-PEG malonic acid diethyl ester (6). NMR (d₆-DMSO): 1.17ppm (t, 6H, —CH₃); 1.99 ppm (quartet, 2H, —CH₂ —CH); 3.21 ppm, (s, 3H,—OCH₃); 3.51 ppm (s, PEG backbone); 4.10 ppm (quintet, 4H, —OCH₂ —CH₃).

M-PEG malonic acid diethyl ester (6) (36 g) was dissolved in 480 ml of1N sodium hydroxide containing 24 g of sodium chloride, and the mixturewas stirred for one hour. The pH of the mixture was adjusted to 3.0 byaddition of 6N hydrochloric acid, and the mixture was extracted withdichloromethane (300 ml and 200 ml). The organic layer was dried overmagnesium sulfate, filtered, concentrated, and poured into cold ethylether. The product M-PEG(20,000)-malonic acid (7) was removed byfiltration and dried under vacuum.

Yield: 32 g. NMR (d₆-DMSO); 1.0 ppm (q, 2H, —CH₂ CH₂CH—); 2.90 ppm (t,2H, —CH₂ CH—); 3.21 ppm (s, 3H, —OCH₃); 3.51 ppm (s, PEG backbone); 12.1ppm (s, 2H, —COOH).

M-PEG malonic acid (7) (30 g) was dissolved in 240 ml of dioxane andrefluxed for 8 hours, then concentrated to dryness. The residue wasdissolved in 200 ml water, extracted with dichloromethane (140 ml and100 ml), dried over magnesium sulfate, and the solution concentrated byrotary evaporation. The residue was precipitated by addition to coldethyl ether.

Yield: 22 g of M-PEG(20,000)-butanoic acid (8). ¹H NMR (d₆-DMSO): 1.72ppm (quintet, 2H, —CH₂ CH₂ CH₂—COON); 2.40 ppm (t, 4H, —CH₂CH₂ CH₂—COOH); 3.21 ppm (s, 3H, —OCH₃); 3.51 ppm (s, PEG backbone). HPLCanalysis showed that the product contained 94 wt % ofM-PEG(20,000)-butanoic acid and 6 wt % of PEG-dibutanoic acid derivedfrom higher molecular weight PEG-diol contained in the starting material

To remove higher molecular weight reactive PEG species,M-PEG(20K)-butanoic acid containing 6 wt % of PEG-dibutanoic acid (22 g)was dissolved in 2200 ml deionized water and applied to a DEAE SephadexA-50 column in the tetraborate form. A stepwise ionic gradient of sodiumchloride (from 1 to 4 mM at increments) was applied, and fractions werecollected. Fractions, containing pure M-PEG(20,000)-butanoic acid, werecombined and collected. Later eluting fractions containing purePEG-dibutanoic acid were set aside. The combined fractions containingpure M-PEG(20,000)-butanoic acid were concentrated (to approx. 200 ml).Sodium chloride (20 g) was added, the pH was adjusted to 3 and theproduct was extracted with dichloromethane. The extract was dried(MgSO₄), and the solvent was distilled off under reduced pressure togive 13.6 g of product.

HPLC analysis showed that the product is 100% pureM-PEG(20,000)-butanoic acid (8) absent higher molecular weight PEGspecies.

C. M-PEG(20,000)-Butanoic Acid, NHS Ester (9)

M-PEG(20,000)-butanoic acid (8) (13.6 g.) was dissolved indichloromethane (40 ml) and N-hydroxysuccinimide (0.094 g) added to thesolution. The solution was cooled at 0° C., and a solution ofdicyclohexylcarbodiimide, DCC, (0.196 g) in 10 ml dichloromethane wasadded dropwise. The solution was stirred at room temperature overnight.The reaction mixture was filtered, concentrated, and precipitated byaddition to ethyl ether.

Yield of final product: 13.1 g. NMR (d₆-DMSO): 1.83 ppm (quintet, 2H,—CH₂ CH₂ CH₂—COO—); 2.70 ppm (t, 2H, —CH₂ —COO—); 2.81 ppm (4H, NHS);3.21 ppm (s, 3H, —OCH₃); 3.51 ppm (s, PEG backbone).

Example 1 Preparation of mPEG (5K)-Thiol MPEG(5K)-CH₂CH₂CONHCH₂CH₂SH(11)

Methoxy-PEG-5K-thiol was prepared in high purity and in high yield froman exemplary electrophilically activated PEG, mPEG-5K propionic acid,N-hydroxysuccinimide (NHS) ester (also referred to as mPEG-5Ksuccinimidyl propionate), commercially available from ShearwaterCorporation, now Nektar Therapeutics, (Shearwater Catalog 2001,Polyethylene Glycol and Derivatives for Biomedical Applications),Huntsville, Ala.

The preparation of mPEG-5K succinimidyl propionate is describedgenerally in U.S. Pat. No. 5,672,662 (Shearwater Polymers), and in the“Materials and Methods” sections above.

Synthesis of M-PEG(5,000)-Thiol (11)

M-PEG propionic acid, NHS ester, (4), (MW=5,268, 10.0 g, 1.898 mmol) wasdissolved in dichloromethane (100 ml) to which were added cystaminedihydrochloride (0.2278 g, 1.012 mmol) and triethylamine (0.66 ml). Thesolution was stirred overnight at room temperature under an atmosphereof argon. The Gel Permeation Chromatography (GPC) analysis showed thereaction mixture contained the desired product (10) (symmetricaldisulfide having a molecular weight of about 10,000) in 97.53% yield andM-PEG(5,000)-Propionic Acid in 2.14% yield.

Dithiothreitol (DTT) (0.88 g, 0.005705 moles) and triethylamine (0.5 ml)were then added and the reaction mixture was stirred for 3 h at roomtemperature under argon. Next 2,6-di-tert-butyl-4-methylphenol (BHT)(0.05 g) was added and the solvent was distilled off under reducedpressure. The crude thiol product (11) was dissolved in dichloromethane(20 ml) and precipitated with isopropyl alcohol at 0-5° C. Yield afterdrying was 8.80 g.

GPC analysis: Desired product: M-PEG(5,000)-thiol, (11), 96.05% yield;M-PEG(5,000)-propionic acid, 0.57% yield; non-reduced dimer, (10),3.07%. NMR (d₆-DMSO): 1.52 ppm (t, 1H, —SH); 2.31 ppm (t, 2H, —CH₂—CO—);2.66 ppm (dt, 2H, —CH₂—S—); 3.21 ppm, (s, 3H, —OCH₃), 3.51 ppm (s, PEGbackbone); 8.05 ppm (t, 1H, —NH—).

Both the exemplary disulfide intermediate, (10), and the reducedPEG-thiol product, (11), were prepared in high yield and in high purityusing a simple reaction scheme requiring only two steps. The exemplaryreagent, cystamine, is commercially available, and the amino grouptherein is readily substituted for the succinimidyl group on thecarbonyl product. The exemplary use of a stoichiometric amount ofsymmetrical reagent having two terminal reactive amino groups makes thereaction “cleaner” due to the formation of only one substitution productnot contaminated by excess of reagent. The resulting PEG-thiol productis suitable for coupling to a reactive thiol group, e.g., contained in adrug or in a cysteine residue of a protein, to form a corresponding PEGconjugate. The PEG linkages (i.e., the linker portion of the moleculeconnecting the PEG chain or backbone to a reactive thiol group on a drugor other species) described herein are stable and provide a new class ofwater-soluble, non-naturally occurring polymers suitable that can bereadily synthesized, and that can be used to selectively modify orpegylate proteins or other reactive molecules without the need formultiple synthetic steps, protection-deprotection steps, and multiplepurifications.

Example 2 Preparation of mPEG (20K)-ThiolCH₃—O—(CH₂CH₂O)_(n)(20K)-CH₂CH₂CH₂CONHCH₂CH₂SH (13)

Methoxy-PEG-20K-thiol was prepared in high purity and in high yield fromanother exemplary electrophilically activated PEG, mPEG-20K butanoicacid, N-hydroxysuccinimide (NHS) ester (also referred to as mPEG-20Ksuccinimidyl butanoate), commercially available from ShearwaterCorporation, now Nektar Therapeutics (Shearwater Catalog 2001,Polyethylene Glycol and Derivatives for Biomedical Applications),Huntsville, Ala.

The preparation of mPEG-5K succinimidyl butanoate is described generallyin U.S. Pat. No. 5,672,662 (Shearwater Polymers), and in the “Materialsand Methods” sections above.

Synthesis of m-PEG(20,000)-Thiol (13)

M-PEG(20K)-butanoic acid, NHS ester (9) (MW=20,000 daltons, 10.0 g,0.500 mmol) was dissolved in dichloromethane (100 ml) and cystaminedihydrochloride (0.0564 g, 0.251 mmoles) and triethylamine (0.167 ml)were added. The solution was stirred overnight at room temperature underan atmosphere of argon. The GPC analysis showed that the reactionmixture contained the desired product (dimer having molecular weightabout 40,000, (12)) 98.5% pure and 1.5% M-PEG(20,000)-butanoic acid.

Dithiothreitol (DTT) (0.23 g, 1.500 mmoles) and triethylamine (0.5 ml)were added and the reaction mixture was stirred for 3 h at roomtemperature under an argon atmosphere. Next BHT (0.05 g) was added andthe solvent was distilled under reduced pressure. The crude product wasdissolved in dichloromethane (20 ml) and precipitated with isopropylalcohol at 0-5° C.

Yield after drying 9.20 g. HPLC analysis: M-PEG(20K)-Thiol (13) 96.0%,M-PEG(20,000)-butanoic acid 1.5%, non-reduced dimer 2.5%.

Similar to Example 1 above, this example demonstrates the preparation ofyet another representative PEG-thiol (as well as its correspondingdisulfide precursor). The synthesis is straightforward, requiring onlytwo reaction steps: substitution of the representative nucleophilicamino group on cystamine at the electrophilic carbonyl carbon on theillustrative PEG reagent, (9), followed by reduction of the disulfide toyield the corresponding PEG-thiol. The use of a symmetrical disulfidereagent simplifies the synthesis, making purification of the PEG-thiolproduct unnecessary. The PEG-thiol is formed in high yield (greater than90%, in fact greater than 95%), and is suitable for coupling withreactive thiol groups e.g., contained in cysteine residues oftherapeutic proteins, or introduced by chemical means into a protein orpolypeptide, or present in small molecules or other active agents.

Example 3 Preparation of PEG(40K)-Di-ThiolHSCH₂CH₂NH(O)CCH₂O-PEG-40K-CH₂C(O)HNCH₂CH₂SH (18)

PEG-40K-di-thiol (18) was prepared from a bifunctional PEG reagent,PEG-40K dicarboxylic acid, as set forth below.

A. PEG (40,000)-Di-Carboxylic Acid, Ethyl Ester, (14)

HO-PEG-OH (MW=40,000 daltons, 50 g, 2.50 hydroxy mequiv.) was dissolvedin 750 ml of toluene and azeotropically distilled for 1 hour under argonatmosphere. 150 ml toluene was distilled from the reaction mixture. Nextthe solution was cooled to 40° C. and 1.0 M solution of potassiumtert-butoxide in tert-butanol (4.0 ml, 4 mmoles) was added, followed byaddition of ethyl bromoacetate (1.4 g, 8.4 mmoles). The reaction mixturewas stirred overnight at room temperature under a nitrogen atmosphere.The solvent was distilled off under reduced pressure, and the crudeproduct dissolved in dichloromethane and added to ethyl ether. Theprecipitated product was isolated by filtration and dried under reducedpressure. Yield 42.3 g.

B. PEG (40,000)-Di-Carboxylic Acid (15)

A solution of 40.0 grams (1.0 mmoles) of PEG (40,000)-dicarboxylic acidethyl ester (14) in 400 ml 1M NaOH was stirred at room temperature for 3hours. Next the pH of the mixture was adjusted to 2 and the product wasextracted with dichloromethane. The solvents were then distilled offunder reduced pressure. The crude product was dissolved indichloromethane (100 ml) and added to ethyl ether (900 ml). Theprecipitated product was isolated by filtration and dried under reducedpressure. HPLC analysis: PEG (40,000)-Di-Carboxylic Acid 86.5%, PEG(40,000)-Mono-Carboxylic Acid 13.0%, HO-PEG(40,000)—OH 0.5%.

Yield 33.1 g. ¹H NMR (d₆-DMSO): 3.51 ppm (s, PEG backbone); 4.02 ppm(4H, —OCH₂COO—).

The obtained product (30 g) was dissolved in 3000 ml deionized water andapplied to a DEAE Sephadex A-50 chromatographic column in thetetraborate form. A stepwise ionic gradient of sodium chloride (from 2to 10 mM at increments) was applied, and fractions were collected.Fractions positive to the PAA test, i.e., containing PEG(40,000)-di-carboxylic acid, were combined and concentrated (to approx.300 ml). Sodium chloride (30 g) was added, the pH was adjusted to 3 andthe product was extracted with dichloromethane. The extract was dried(MgSO₄), and the solvent was distilled off under reduced pressure togive 21.6 g of product, (15). The product (15) was shown to be puredi-acid, i.e., 100% PEG (40,000)-di-carboxylic acid, by HPLC.

C. PEG(40,000)-Di-Carboxylic Acid, NHS Ester, (16)

PEG (40,000)-di-carboxylic acid (15) (20 g) was dissolved indichloromethane (200 ml) to which was added N-hydroxysuccinimide (0.138g). The solution was cooled to 0° C., a solution ofdicyclohexylcarbodiimide (0.290 g) in 5 ml dichloromethane was addeddropwise, and the solution was stirred at room temperature overnight.The reaction mixture was filtered, concentrated, and precipitated byaddition to ethyl ether. Yield of final product, (16): 18.3 g.

¹H NMR (d₆-DMSO): 2.83 ppm (8H, NHS); 3.51 ppm (s, PEG backbone); 4.61ppm (4H, —OCH₂COO—).

D. PEG(40,000)-Di-Thiol (18)

PEG(40,000)-di-carboxylic acid, NHS ester (16) (MW=40,000, 18.0 g, 0.900mequiv.) was dissolved in dichloromethane (150 ml) and cystaminedihydrochloride (1.01 g, 4.5 mmoles) and triethylamine (1.70 ml) wereadded. The solution was stirred overnight at room temperature underargon atmosphere. The solution was concentrated and added to 900 ml ofisopropyl alcohol at room temperature. The precipitated product wasremoved by filtration and dried under reduced pressure.

NMR analysis showed that all NHS ester was consumed and the desireddisulfide product (17) was formed. The product was dissolved indichloromethane (150 ml), dithiothreitol (DTT) (0.84 g, 5.446 mmoles)and triethylamine (2.0 ml) were added and the reaction mixture wasstirred 3 h at room temperature under an argon atmosphere. Next BHT(0.09 g) was added and the solvent was distilled off under reducedpressure. The residue—a crude dithiol product (18) was dissolved indichloromethane (40 ml) and precipitated with isopropyl alcohol at roomtemperature. Yield after drying 14.3 g.

¹H NMR (d₆-DMSO): 1.07 ppm (t, 2H, —SH); 2.66 ppm (dt, 4H, —CH₂—S—);3.51 ppm (s, PEG backbone); 3.90 ppm (s, 4H, —OCH₂CO—, 8.05 ppm (t, 2H,—NH—).

What is claimed is:
 1. A polymer having the following structure:POLY-L_(0,1)-X—Y—S—S—Y—X-L_(0,1)-POLY wherein: POLY is an end-cappedpolyalkylene oxide; L is an optional linker; X is selected from thegroup consisting of amide, carbamate, carbonate ester, ether andthioester; Y is selected from the group consisting of alkylene,substituted alkylene, cycloalkylene, substituted cycloalkylene, aryl,and substituted aryl, comprising from about 2 to about 10 carbon atoms;and S is a sulfur atom.
 2. The polymer of claim 1, wherein L₁ is alinker selected from the group consisting of C₁-C₁₀ alkyl and C₁-C₁₀substituted alkyl.
 3. The polymer of claim 1, wherein L₁ is a linkerselected from the group consisting of (CH₂)_(1, 2, 3, 4 and 5).
 4. Thepolymer of claim 1, wherein L is —CH₂—.
 5. The polymer of claim 1,wherein POLY is a polyethylene glycol having the structureH₃CO—(CH₂CH₂O)_(n)CH₂CH₂— wherein n ranges from 10 to about 4,000. 6.The polymer of claim 1, wherein L is absent (L₀).